The congressional testimony of Trump-Russia figure Carter Page shows that a Democratic-financed dossier can dominate how FBI agents question witnesses.

Mr. Page testified Nov. 2 before the House intelligence committee, which released a transcript Monday evening.

Rep. Trey Gowdy, South Carolina Republican, was particularly interested in how the FBI is conducting its probe of supposed Russian collusion with the Trump campaign.

“So what were the questions the FBI had for you?” Mr. Gowdy said.

“It was an extensive series of many meetings, but they, you know, the core foundation, I would say, in terms of the overall structure in March 2017, was related to those false allegations from the ‘dodgy’ dossier,” Mr. Page said, using his moniker for the anti-Trump document.

Mr. Page has been perhaps the most outspoken Trump figure caught up in the congressional and special counsel’s investigation into Russian interference in the 2016 election. He has made numerous TV appearances and has released several letters and statements.

He said the dossier and associated investigations have ruined his energy firm. He has no business income, he said, and is living off investments as he worries about death threats and moves around a lot.

The dossier was funded by the Democratic Party and the Hillary Clinton campaign. They paid the private intelligence firm Fusion GPS, which hired British ex-spy Christopher Steele, to investigate and produce it. He wrote the dossier based on paid sources inside the Kremlin. The coordination prompted conservatives to charge that it was the Democrats who actually colluded with the Russians to influence the election.

Mr. Page, who lived and worked in Moscow in the 2000s as a Merrill Lynch banker, is one of Mr. Steele’s main targets in the dossier.

Mr. Steele accuses him of several felonies, such as meeting with sanctioned Kremlin figures while in Moscow in July 2016 to address the New Economic School. Mr. Steele said Mr. Carter promised to work to end U.S. sanctions against Russia in exchange for a brokerage commission. The dossier also said he and former Trump campaign Chairman Paul Manafort orchestrated the Russian interference to include hacking Democratic Party computers.

Mr. Page, a Trump campaign volunteer on national security, says that all of those charges are fiction. He never met the two Kremlin operatives and has never met Mr. Manafort.

For months Republicans have tried to learn the extent to which then-FBI director James B. Comey relied on the dossier to open and conduct an investigation of the Trump campaign in the summer of 2016. For example, did Mr. Steele’s unverified charges underpin the FBI’s request for eavesdropping warrants?

Mr. Comey refused to discuss the dossier in open testimony.

Last week, the FBI provided classified documents requested by House Permanent Select Committee on Intelligence Chairman Devin Nunes, California Republican. A spokesman said the information remains classified and is not releasable.

The FBI has had Mr. Page under surveillance to the point of following him around New York.

Asked at the Nov. 2 hearing how many times he sat down with the FBI, Mr. Page said, “Approximately four.