Sen. Al Franken on CNN. CNN Democratic Sen. Al Franken on Sunday doubled down on a suggestion that some of his Republican colleagues in the Senate are concerned about President Donald Trump's mental health.

In an interview on CNN's "State of the Union" on Sunday, host Jake Tapper asked Franken if he was serious when he told HBO's Bill Maher that some of his Republican colleagues "will say [Trump's] not right mentally, and then some are harsher."

"Yes," Franken said. "It's not the majority of them. It's a few."

The Minnesota senator explained that some Republicans were puzzled and worried by Trump's continued insistence — despite no evidence — that millions of people voted illegally against him in states like California and New Hampshire. The president asserted last week in a meeting with senators that thousands of out-of-state voters were bused into the state, resulting in his defeat in the state during the 2016 election, a claim that he backed with no evidence.

"We all have this suspicion that — he lies a lot. He says things that aren't true. That's the same as lying, I guess," Franken said, before noting Trump's voter fraud claims. "'Three million to five million people voted illegally.' There was a new one about people going in from Massachusetts to New Hampshire."

He added: "That is not the norm for a president of the United States, or, actually, for a human being."

Despite a lack of any corroborating evidence, The Trump administration didn't back down from the president's new voter fraud claims.

During a testy interview on Sunday, ABC anchor George Stephanopoulos appeared agitated after repeatedly attempting, and failing, to elicit specific evidence from Trump adviser Stephen Miller to support the president's claims.

"Just for the record, you have provided absolutely no evidence," Stephanopoulos said.

"The White House has provided enormous evidence," Miller interjected.

He continued: "George, it is a fact and you will not deny it, that are massive numbers of non-citizens in this country who are registered to vote. That is a scandal. We should stop the presses and as a country we should be aghast about the fact that you have people who have no right to vote in this country registered to vote, canceling out the franchise of lawful citizens of this country."

Senate Health, Education, Labor, and Pensions Committee member Sen. Al Franken, D-Minn. speaks on Capitol Hill in Washington, Tuesday, Jan. 31, 2017, during the committee's executive session on the nomination Education Secretary-designate Betsy DeVos. ( AP Photo/Alex Brandon

Despite his frequent assurances that he is not interested in seeking higher office, Franken's political prospects have become the subject of fascination among many Washington pundits, as he's emerged as a critic of Trump's cabinet nominees and policies.

Outlets including the Washington Post and the National Journal last week both published op-eds touting the Minnesota senator's potential strengths in a hypothetical presidential matchup with Trump, citing his history as a comedian and appeal to white voters who backed Trump over Hillary Clinton in the 2016 presidential campaign.

Other outlets like CBS News and the New Republic suggested that Franken's viral grilling of Trump's cabinet nominees showed he could be a powerful force in the Democratic party.

Many Democratic strategists and staffers have privately speculated about Franken's prospects, but aren't certain of his intentions.

It's "way too early," former Minneapolis Mayor RT Rybak told Business Insider last week. "I love Al as my senator, but I imagine there will be thousand of boomlets in the next couple years, and about the only thing we should take seriously for the next year is how to stop Trump and win the midterms."