Republican leaders of the Senate Judiciary Committee have asked the Justice Department to investigate whether Mr. Steele, whom Mr. Simpson hired, lied to federal authorities about his contacts with journalists.
Mr. Simpson himself has been hauled before three congressional committees for some 20 hours of questions and answers, placing him among the most significant players in the Trump-Russia affair, if math is the metric.
“Uncooperative,” Senator Charles E. Grassley, Republican of Iowa, said of Mr. Simpson’s turn before the Judiciary Committee, which he leads.
“Very cooperative,” said Representative Adam B. Schiff of California, the top Democrat on the House Intelligence Committee, where Mr. Simpson also appeared.
Mr. Simpson can be both of these things — according to interviews with dozens of friends, colleagues and investigative targets — and a few more: brash, obsessive, occasionally paranoid, perhaps with cause.
The name of the firm, founded in 2010, was intended to represent a “fusion” of journalism and business intelligence. The “GPS” hinted aspirationally at a global reach.
In practice, Fusion’s task has often translated, roughly, to finding unsavory things about unsavory people, at the behest of not-especially-savory clients. The firm often represents corporations, hedge funds or law firms, providing a sort of public-records forensics that resembles journalism. It leans on its understanding of the news media, and its contacts among reporters, to elevate its clients and squeeze their adversaries.
In election years, political opposition research can consume more of Mr. Simpson’s time, with bipartisan demand.
The pay is good. And the targets are rich.
“He’s trying to figure out the puzzle,” said Stuart Karle, a friend and the former general counsel for The Journal. “That animates him. It always has.”
Yet Mr. Simpson’s roster of enemies is legion, and sometimes contradictory, powered by accusations that he has played both sides of the Russia divide — and of American politics. During the campaign, anti-Trump forces from both parties by turns bankrolled Fusion’s research.
More notable to some lawmakers is Mr. Simpson’s connection to two figures with deep bonds to Russian power, who have long fought for a pet Kremlin cause: Rinat Akhmetshin, a Russian-American lobbyist, and Natalia V. Veselnitskaya, a lawyer known as a formidable Moscow insider.
The two have worked to turn back the Magnitsky Act, a 2012 American law detested by President Vladimir V. Putin of Russia, that punishes Russian human rights violators. And together with Mr. Simpson, the trio worked on behalf of a Russian-owned company accused by the United States government of laundering some of the funds stolen in a fraud uncovered by Sergei L. Magnitsky, after whom the act was named.
Mr. Simpson had been hired by a law firm, BakerHostetler, representing the company. As part of this work, he compiled damaging allegations against William F. Browder, an American-born financier and Kremlin foe who was the driving force behind the Magnitsky Act.
“He’s a professional smear campaigner and liar for money,” Mr. Browder said of Mr. Simpson. “The credibility of anything that he does is in question.”
The Magnitsky Act was a topic of discussion at an infamous meeting that both Ms. Veselnitskaya and Mr. Akhmetshin attended at Trump Tower in June 2016 with Donald Trump Jr., who had been promised compromising information on Hillary Clinton.
Mr. Simpson was in court with Ms. Veselnitskaya hours before the Trump Tower meeting and saw her again shortly after it, his lawyer, Joshua A. Levy, confirmed. But these contacts were related to Mr. Simpson’s work on the case for BakerHostetler, Mr. Levy said, and Fusion only learned about the Trump Tower meeting in news reports last year.
“We collect facts,” Fusion said in a statement, describing itself as a research company. “Occasionally, the facts turn out to be helpful to people we deplore, like Vladimir Putin, or undermine people for whom we have considerable sympathy, like William Browder.”
For his efforts, Mr. Simpson has been presented with a defamation lawsuit and a congressional subpoena demanding bank records, which the firm is fighting in court, saying it could jeopardize a confidential client list in an industry where privacy is paramount. A legal defense fund has been formed, one of Fusion’s lawyers, Theodore J. Boutrous Jr., said, adding that the firm’s legal costs had by now outstripped all compensation for investigations into Mr. Trump and his links to Russia during the campaign.
But the work has not stopped. Fusion continues to look into ties between Mr. Trump and Russia, according to several people briefed on the research. Mr. Simpson’s specific areas of focus, and information about any current benefactors, are closely guarded.
“Glenn’s a relentlessly digging sort of guy,” said Tim Burger, a friend and former colleague. “If there is a document out there about anything, he will find it.”
The Spy and the Scribe
The marriage made enough sense on paper: the former spy and the former scribe, feeling their way through second acts of lives spent investigating furtive actors who preferred to be left alone.
Mr. Steele, now in his 50s, had worked undercover in Moscow in the 1990s, later serving as the top expert on Russia at the London headquarters of Britain’s spy service, MI6.
Like Mr. Simpson, he had changed careers in 2009, opening his own commercial intelligence firm.
Like Mr. Simpson, according to people who know them, he came to hold a dark view of Mr. Putin.
Since shortly after Mr. Trump’s candidacy began, Fusion had been researching his past scandals and possible vulnerabilities, gathering documents on his time in business and entertainment. The firm was paid initially by a conservative website, The Washington Free Beacon, financed largely by Paul Singer, a top Republican donor who had opposed Mr. Trump.
As Mr. Trump emerged as the presumptive Republican presidential nominee, Democrats began picking up the tab. A law firm, Perkins Coie, hired Fusion to gather information about Mr. Trump on several fronts, including possible ties to Russia, on behalf of Mrs. Clinton’s campaign and the Democratic National Committee.
Soon, a new urgency set in. By June, there were reports that the committee had been hacked, apparently by Russian agents. Stolen documents began appearing online.
Mr. Simpson connected with Mr. Steele, whom he had met in 2009, when they were introduced by a mutual acquaintance who knew of their shared interest in investigating Russian corruption. They quickly got to work.
As a former spy inside Russia, Mr. Steele was ill positioned to travel there himself. He hired native speakers to call informants inside the country and communicated discreetly with his own connections.
His findings — more a series of memos than a single narrative arc — outlined a yearslong effort to influence Mr. Trump, perhaps initially because he had intersected with Russian oligarchs whom Mr. Putin hoped to track. The dossier’s most memorable entry is an unsubstantiated account involving Mr. Trump, Russian prostitutes and a defiled Moscow hotel bed where the Obamas once slept.
The research also described a series of contacts between Trump associates and Russians during the campaign, in part to discuss the hacking of the Democratic National Committee and John D. Podesta, Mrs. Clinton’s campaign chairman.
In time, Mr. Steele shared some of his information with the F.B.I.
“He felt he was witnessing a national security crime in progress,” said Mr. Simpson, who declined to be interviewed for this article but addressed some questions via email. “I did not object, and did not ask for permission from our clients. Chris was the national security professional, so I deferred to him.”
Though Mr. Trump and his allies have suggested that the dossier alone prompted the F.B.I.’s investigation into whether his campaign colluded with the Russians, current and former American and foreign officials have said otherwise. The primary impetus came, instead, with the revelation that a Trump foreign policy adviser told an Australian diplomat that Russia had political dirt on Mrs. Clinton, coupled with the release of the hacked Democratic National Committee emails.
Still, by late in the year, the dossier was Washington’s most notorious open secret. Reporters, including some from The New York Times, worked to confirm or disprove the memos. Both Mr. Trump and President Barack Obama were briefed on the material during the transition.
By then, Mr. Simpson’s period of relative anonymity had come and gone, replaced by a spotlight that has exacerbated the kind of distrust endemic to his line of work.
His name was back in the paper the next day. It had been awhile since that happened against his will.
A Tumultuous Life
Mr. Simpson was a high school senior, not yet 18.
It was nearly graduation for Conestoga High School’s Class of 1982, and Mr. Simpson capped the first day of spring at his family’s home in a Philadelphia suburb with a Saturday night party, stocked with alcohol. After an evening of hard drinking, a friend staggered outside and into the road. He was struck by a car and killed.
At once, a reckoning gripped the region. Mr. Simpson’s mother, Jane, was charged with allowing her son to hold the party. A front-page article in The Philadelphia Inquirer, centered on Mr. Simpson’s gathering, detailed the rise of binge drinking. “It’s a release,” Mr. Simpson was quoted telling the newspaper. “It’s a way to forget.”
The most serious charges against Mrs. Simpson were dismissed. But the episode was an early signpost of an often tumultuous life for her son, vacillating from swashbuckling fact-finding to bursts of trauma and back again.
In his early 20s, as a student at George Washington University, Mr. Simpson broke his neck in a car crash. In his late 30s, he turned back spinal cancer.
In between, Mr. Simpson produced some of Washington’s most important journalism, writing stories on the Keating Five savings-and-loan scandal and the assorted misadventures of Newt Gingrich, then the House speaker. He earned a reputation for dogged, document-based foraging. C-Span 2 once devoted 17 unremarkable minutes to a segment on Mr. Simpson rummaging around the public records room at the Federal Election Commission.
“He had a very strong sort of ethical backbone,” said Jim Glassman, the former editor in chief of Roll Call, a publication dedicated to covering Congress and Mr. Simpson’s professional home before he joined The Journal. “He was a terrific reporter, kind of a prodigy.”
Before long, the book industry beckoned. Mr. Simpson’s “Dirty Little Secrets,” written with Larry J. Sabato, focused on political corruption.
One section tackled the burgeoning field of opposition research: “There is relatively easy money to be made,” it read. “The scrupulous and unscrupulous alike are attracted to offer their wares.”
By the time he reached The Journal in 1995, Mr. Simpson had assumed an eclectic place among the capital’s constellation of influencers — a hard-charging reporter with a deep Rolodex, a weakness for Bob Dylan and an unquenchable thirst for revelry, mischievous or otherwise. The weekend of his Arkansas wedding to Mary Jacoby, who was also at Roll Call, included skeet shooting, chicken fried steak and a write-up previewing the festivities in The Washington Post.
His home in Washington’s Friendship Heights became a rollicking hub for journalists blowing off steam, with sporadic appearances from political power brokers. His Journal desk became a gathering place for half-eaten sandwiches and burps he would not bother stifling.
After the Sept. 11, 2001, attacks, Mr. Simpson turned his attention abroad, eventually relocating to Brussels, pursuing stories on the financial sources of terrorism. He was sued repeatedly in foreign courts. The Journal suffered losses in legal fees, if not credibility, on his account.
“Glenn Simpson changed 800 years of English libel law,” Mr. Karle, the paper’s former general counsel, said of one case. “It was the first significant decision from the House of Lords that found that a journalist had behaved well.”
By the late 2000s, the ground at The Journal, and across journalism, had begun to shift. Rupert Murdoch assumed control of the newspaper. The industry, generally, allowed less latitude for the kinds of costly, long-term investigations that were Mr. Simpson’s trade.
In 2009, Mr. Simpson announced he was leaving the profession to start SNS Global, a research company that predated Fusion, with a Journal peer, Susan Schmidt. On his last day, former reporters there recalled, Mr. Simpson brought his dog, a dachshund-beagle mix named Irving, to join him in the office, loudly encouraging him to defecate near some editors. (It is not clear if Irving obliged.)
But before he was gone, Mr. Simpson had wandered down rabbit holes that would prove instructive.
Stories about the Russian power nexus in Washington became a fascination. So did the foreign entanglements of a well-known lobbyist, referenced repeatedly under Mr. Simpson’s byline: Paul Manafort, the now-indicted former campaign chairman for Mr. Trump.
A Conservative Byword
The name has not helped.
Opaque and sinister sounding, with an almost ostentatiously spare website to match, “Fusion GPS” has quickly joined the pantheon of conservative media buzz-phrases, like “Benghazi” and “Clinton Foundation” before it, signaling liberal misdeeds and cover-ups.
“FUSION GPS SCANDAL,” read a recent headline from Sean Hannity on Fox News, stretching across the top of a studio monitor. From this screen, he connected a series of rectangles with short white lines to allege a Fusion-powered megaconspiracy against Mr. Trump.
The attention has become more than a distraction. Mr. Trump himself has pressed at least one Republican senator to open an investigation into Fusion’s connections to Democrats, and the argument has found a receptive audience with some conservatives in Congress.
Mr. Grassley, the Senate Judiciary Committee chairman, has also raised concerns about Fusion’s compliance with the Foreign Agents Registration Act, a tactic that Fusion’s allies have dismissed as a sideshow to protect the president. Fusion has said it is not a lobbying firm and has followed all laws. In an editorial last week, Mr. Simpson and his co-founder at Fusion, Peter Fritsch, another Journal veteran, called on Congress to release the transcripts of the firm’s testimony “so that the American people can learn the truth about our work and most important, what happened to our democracy.”
Yet Fusion’s typical mandate, even defenders acknowledge, is not dripping with nobility.
Few Washington industries traffic in sainthood. Many are mercenary in nature. And Fusion seems to be of a piece, hustling for its cut from an office in Dupont Circle.
The firm said it had “worked for both Republicans and Democrats in a handful of elections.” The Washington Post has reported that research targets included tech giants like Google and Amazon and both major-party presidential nominees in 2012, Mitt Romney and Mr. Obama.
Mr. Simpson also conducted work for Planned Parenthood as the group navigated the fallout from videos that appeared to suggest the organization was profiting from fetal tissue sales. Mr. Simpson found that the videos had been edited misleadingly. Conservatives have cited this project as evidence of bias toward liberal causes.
Amid skepticism over the dossier, Fusion has trumpeted Mr. Steele’s strong standing in the intelligence community, noting that the former spy is trained to screen out disinformation.
Still, at a firm that casts itself as a collector of facts, the dossier is not the equivalent of a finished news article, in journalism parlance. It is more a compilation of prepublication notes, requiring further vetting.
Mr. Steele has reportedly appraised the memos as 70 to 90 percent accurate, an estimate that Mr. Simpson is said to embrace himself. Less clear is precisely which 10 to 30 percent should be disbelieved.
But Mr. Simpson has bristled at the idea that the dossier is some kind of liberal fever dream. Its broad conclusions — once outlandish themselves — have already proved prescient, the firm has argued. Sometimes the conspiracy is true.
“More than a year has gone by since these memos were produced,” Mr. Simpson said. “I have not become aware of any disinformation.”