Glenn Simpson, key figure behind million-dollar 'dossier,' to face questions Glenn Simpson of Fusion GPS will be questioned on Capitol Hill on Tuesday.

 -- A key figure behind the so-called dossier featuring uncorroborated and salacious allegations about then-candidate Donald Trump’s ties to Russia will be questioned by investigators from the Senate Judiciary Committee today about the funding and sources for the document.

During last year’s heated Republican primary race, Fusion GPS, a private research firm founded by former Wall Street Journal reporter Glenn Simpson, was initially paid about a million dollars by wealthy Republicans and then later worked for Democrats, all of whom wanted to dig up dirt on Trump and plant negative news stories, according to political operatives.

Simpson, who will appear in a closed session on Capitol Hill on Tuesday, hired the former MI6 agent Christopher Steele to compile the now infamous dossier, which alleged that members of the Trump campaign colluded with Russian agents to damage Hillary Clinton, Trump’s Democratic opponent.

Republicans in Congress are stepping up their efforts to uncover the funders of and sources for that controversial document and its — so far — largely unverified claims as special counsel Robert Mueller’s high-profile probe of those alleged ties heats up.

Sen. Chuck Grassley, R-Iowa, the chairman of the Senate Judiciary Committee, has made it clear that Simpson’s work has landed him in the crosshairs.

“We will also pursue details about Mr. Simpson’s role in this event and the creation and circulation of the dossier that started this whole controversy,” Grassley said in a hearing in July.

As a reporter, Simpson specialized in coverage of money laundering and Russian organized crime. In an appearance on a panel at a film festival in 2016, he explained that he started Fusion GPS after leaving journalism because he thought his investigative skills would be valuable to a range of wealthy clients.

“What I really like to do is gather documents and put things together in a way that they could be used to expose a crime or right a wrong,” Simpson said. “I call it journalism for rent.”

The 35-page dossier, which included a series of salacious allegations, was completed shortly before the presidential election. Eventually the intelligence Steele gathered was shared with journalists, the Clinton campaign and the FBI.

When BuzzFeed obtained and published the document in January, little more than a week before of Trump’s inauguration, he was outraged.

“It’s all fake news. It’s phony stuff. It didn’t happen,” Trump said at a press conference. “And it was gotten by opponents of ours, as you know, because you reported it and so did many of the other people. It was a group of opponents that got together — sick people — and they put that crap together.”

Simpson isn’t the only person who could face questions about dossier’s production. Lawmakers also want to talk to Steele, and a recent U.S. court ruling in a related case could compel him to comply.

Lawyers for Aleksej Gubarev, a Russian tech mogul who was named in the dossier, were recently granted approval by Judge Ursula Ungaro of U.S. District Court in the Southern District of Florida to seek British approval to question Steele under oath as part of Gubarev's libel suit against BuzzFeed.

“I have to ask myself, ‘What is it that they’re hiding?’” said Valentin Gurvits, Gubarev’s attorney. “To me, it is a very strange situation.”

According to people briefed on the developments, Steele has met with the FBI and provided agents with the names of his sources for the allegations in the dossier.

It is unclear how much information lawmakers will be able to obtain from Simpson this week. Attorneys for Fusion GPS have indicated to the committee that its client relationships are confidential.

ABC News’ Randy Kreider contributed to this report.