WATCH: Top House Democrat makes wild claim about widely panned Steele dossier that leaves Fox News host stunned

Fox News host Martha MacCallum grilled Rep. Eric Swalwell (D-Calif.) over his insistence that President Donald Trump is guilty of colluding with Russia to undermine the 2016 presidential election despite the conclusions of special counsel Robert Mueller's investigation.

Swalwell's name was included on a list of Democratic politicians and commentators the Trump campaign circulated Monday urging TV networks to force the politicians to answer for their "false claims" about Trump.

On Tuesday, MacCallum did just that, grilling Swalwell for maintaining that the unverified, salacious dossier compiled by former British spy Christopher Steele contains an iota of truth, in addition to his claim last year that Trump is a "Russian agent."

What did Swalwell say about the dossier?

"Which part of [the dossier] hasn't been proved factual?" Swalwell said.

The question — and the underlying suggestion, being that the dossier is factual — stunned MacCallum.

"I mean ... are you serious?" she said bewildered.

After MacCallum answered Swalwell's question, highlighting several of the dossier's claims that remain unfounded more than two years after its compilation, Swalwell shifted the conversation to the integrity of Michael Cohen, Trump's former personal lawyer.

"Well, you can be on record as standing by what was found in the dossier," MacCallum said.

Rep. Swalwell: The only person caught lying about Russia is the presidentyoutu.be

How did Swalwell respond to his claim that Trump is a Russian 'agent'?

"Do you think that was too strong a word to call the president of the United States of America a Russian agent," MacCallum pressed. "That is a very strong charge, and you said you have evidence to back that up. Do you stand by that?"

In response, Swalwell said he believes Trump "acts on Russia's behalf," citing Trump's desire to diminish the roll of NATO and his previous desire to withdraw U.S. troops from Syria.

MacCallum, dissatisfied that Swalwell maintains his belief that Trump has acted on behalf of Russia's interests, suggested Swalwell should engage in retrospection in light of Mueller's conclusions.

"I just feel like that in light of the Mueller finding, that maybe it's time for a little bit of retrospective [sic] of what you said in the past," she suggested.

"I don't think you want me to lay out the evidence," Swalwell responded, "because I can lay out the evidence."

"Go ahead. Mueller laid out 2,800 subpoenas and two years of evidence, but if you got something that he didn't find then let's hear that," MacCallum said, reiterating her suggestion for retrospection.

"Donald Trump acts at Russia's behalf," Swalwell responded defiantly, later explaining that he does not regret any of his claims.

Rep. Swalwell: The only person caught lying about Russia is the presidentwww.youtube.com