In nearly seven hours of testimony before the House Intelligence Committee, Simpson defended the Trump opposition research dossier compiled by Christopher Steele, arguing that nothing from the dossier had been disproven and some of it had been corroborated, the sources said.
Republicans on the committee pushed back during the interview that many details in the dossier have not been verified -- they said that it's often difficult, if not impossible, to prove a negative, according to the sources.
Simpson added that he is still in regular contact with Steele, who has yet to cooperate with congressional investigators.
Simpson's testimony is the latest volley in the swirl of controversy surrounding the Steele dossier, which has been slammed as a "hoax" by the President.
Trump and his Republican allies have seized upon the disclosure last month
that law firm Perkins Coie paid for the dossier on behalf of its clients, the Hillary Clinton campaign and the Democratic National Committee, arguing it shows the dossier is a Democratic effort to undermine Trump.
"Clinton campaign & DNC paid for research that led to the anti-Trump Fake News Dossier. The victim here is the President," Trump tweeted last month.
The broad assertion of the dossier findings -- that Russia waged a campaign to interfere in the election -- is now accepted as fact by the US intelligence community. CNN also reported earlier this year that US investigators have corroborated some aspects of the dossier, specifically that some of the communications among foreign nationals mentioned in the memos did actually take place.
The FBI also used some information as part of its justification to obtain a surveillance warrant on Trump campaign foreign policy adviser Carter Page, CNN has reported. But federal investigators have not verified the dossier's most salacious allegations.
Several of the Trump associates named in the dossier have disputed allegations made about them in the dossier, including Page and Trump's personal lawyer Michael Cohen. Cohen has pointed to the allegation in the dossier that he traveled to Prague, Czech Republic, in August 2016 to meet with Russian officials, saying he never made a trip there. No evidence has emerged to support the claim Cohen traveled to Prague in August 2016.
The Steele memos also accuse Page of meeting with the president of the Russian state-run oil giant Rosneft. He vehemently denies ever meeting that official, Igor Sechin, though he acknowledged in testimony earlier this month that he met with a different executive from Rosneft.
After publishing the dossier, BuzzFeed was sued by Aleksej Gubarev, a Russian CEO who is named in the document. Gubarev denied the claims made about him in the dossier. BuzzFeed redacted Gubarev's name from its published version of the dossier and issued an apology for including it shortly after the lawsuit was filed.
California Rep. Adam Schiff, the top Democrat on the committee, argued that the key conclusion from the Steele dossier has been proven, despite the White House's attacks.
"The reality is that on the most important conclusion of the dossier and Christopher Steele -- that is that the Russians were actively intervening in the election to help Donald Trump -- that conclusion has now been borne out, and Christopher Steele found evidence of that before our own intelligence agencies potentially were aware of that," Schiff said.
Republicans were pressing to learn who funded the Steele dossier, and the House Intelligence Committee is still battling with Fusion GPS in court over the firm's bank records.
Simpson's testimony before the panel seemed unlikely only a week ago in response to the committee's subpoena, as two other Fusion GPS partners had invoked their constitutional rights not to answer questions from Congress.
But Simpson and the committee struck an agreement last week for him to testify voluntarily, so long as he maintained his right not to answer questions that would violate confidentiality agreements with clients.
The sources said Simpson was initially hesitant to answer the committee's questions, but he eventually opened up with his responses.
Rep. Mike Conaway, the Texas Republican leading the committee's Russia probe, said there were still some questions he declined to answer involving specific names.
"There were things that he was unwilling to answer based on a confidentiality agreement with his client," Conaway said. "And because it was on a voluntary basis, we couldn't push the issue."
A Fusion GPS spokesperson referred questions about Simpson's testimony to a statement the group's lawyer issued Tuesday evening, saying Simpson is cooperating with the three congressional committees investigating Russian meddling.
Joshua Levy, a lawyer for Fusion GPS, said in a statement that "Mr. Simpson and Fusion GPS are proud to have maintained the confidentiality of their clients throughout this past year in the face of false accusations and pressure to violate that obligation."
"The White House and its political allies have attempted at every turn to smear Fusion GPS because of its connection to the Steele dossier, whose findings are the subject of multiple investigations into the Trump campaign's collusion with the Russians," Levy said.
The court fight is still ongoing, with Fusion GPS, TD Bank and House lawyers appearing in federal court Wednesday afternoon. Judge Richard Leon is weighing whether the court should stop the bank from releasing Fusion GPS' financial information to Congress. Leon encouraged Fusion GPS, TD Bank and the House to unseal more of their case and make it available to the public and set a time for them to argue over the subpoena on November 30.
In late October, the House intelligence committee, led by Rep. Devin Nunes, a California Republican, subpoenaed TD Bank for all financial records related to Fusion GPS. The political intelligence firm balked at the breadth of the request.
Two Fusion GPS clients -- the Democratic Party's law firm Perkins Coie and the right-leaning news organization the Washington Free Beacon -- then revealed themselves as funding opposition research on Trump during the 2016 presidential campaign. Fusion GPS said it paid Steele $168,000 for his work on the dossier that's become a focal point of questions related to Trump and the investigation into the campaign's collusion with Russia. Trump has denied the allegations in the dossier.
The fight over the bank's release of records resurfaced on November 3, when Fusion GPS said the House committee again asked for more than a quarter of the 400 bank records, including information about any law firms, media companies and journalists with which it worked.
More specific details about the House's request and its response to the renewed lawsuit are still under seal with the court.
UPDATE: This story has been updated with Rep. Schiff's comments.