WATCH: Chris Matthews crushes Rudy Giuliani for claiming that Trump gave up 'birtherism'

MSNBC host Chris Matthews hammered Donald Trump surrogate Rudy Giuliani on Thursday when Giuliani tried to argue that the GOP presidential nominee had renounced his "birther" attacks against President Barack Obama.

"Can you commit to your candidate saying in the next 24 hours that President Barack Obama's a legitimate president?" Matthews asked. "Can you commit for him that he will say that?"

"Just go back and look -- he has said it already," Giuliani claimed.

"No he hasn't," Matthews said after a second. "You are wrong on the fact, mister mayor."

"But I think he did," Giuliani replied, without identifying a source for his statement.

"I think you're wrong," Matthews said, apparently stifling a chuckle.

The former New York City mayor then claimed -- falsely -- that Hillary Clinton's campaign was behind the conspiracy theory painting Obama as being born in Kenya, which would have rendered him ineligible for the presidency.

As Politifact reported last year, that rumor began with a Clinton volunteer, Linda Starr, who brought her allegations to an attorney in Pennsylvania for a lawsuit against Obama that was ultimately thrown out.

"There is no record that Clinton herself or anyone within her campaign ever advanced the charge that Obama was not born in the United States," Politifact stated at the time. "A review by our fellow fact-checkers at Factcheck.org reported that no journalist who investigated this ever found a connection to anyone in the Clinton organization."

For his part, Trump -- who was roundly mocked in 2012 for offering $5 million to Obama if he could "prove" his US citizenship (even though he already had) -- has taken to saying, "I don't talk about it anymore" when asked about the issue during his current presidential campaign.

Regardless, Giuliani said, "Donald Trump believes that [Obama] was born in the United States," prompting Matthews to shoot back, "When's he gonna say it? When is he going to say that this president is legitimate? This is a fundamental question, mister Mayor: Is the president of the United States legitimate or not? If you believe it, why doesn't your candidate state it?"

Watch the interview, as aired on Thursday, below.