Speaker Mike Johnson is expressing confidence that Republicans have enough momentum to win the White House in November – no matter who the Democratic presidential candidate is.

"As President Trump has said, he was, they had sort of prepared in the mindset that they would run against Biden, but it doesn't matter. I mean, if they put Kamala Harris at the top of the ticket, she's the co-owner of all the policies, it's not any better," the Louisiana Republican told Fox News Digital in a Thursday interview.

"It doesn't matter who they run. Anybody that they would put in that place – this election is not about personalities, it's about policies and what it means to people."

It comes as Democratic pressure continues to build on President Biden to drop his re-election bid.

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Former President Donald Trump and House Speaker Mike Johnson

Speaker Mike Johnson said he's confident Donald Trump could beat whomever Democrats run as their presidential candidate. (Getty Images)

The 81-year-old president is facing calls to duck out of the race after his disastrous debate performance last month. It’s brought out concerns that Biden may not have the physical or mental stamina to run for office nor hold it for another four-year term.

Johnson, who has long accused Biden of not having the mental acuity to hold office, would not say whether the president should leave office right now. But he pointed out that even senior Democrats are apparently moving behind the scenes to push Biden off the 2024 ticket.

"Look, it's for him to determine. They're in real turmoil. On the other side, you hear in the last 24 hours, I'm told that [Senate Majority Leader Chuck Schumer, former Speaker Nancy Pelosi and House Minority Leader Hakeem Jeffries] have all pretty well indicated that he – told him, I guess, or at least implied publicly that he should… not run for re-election," Johnson said. 

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President Biden and former President Trump debate at CNN Studios on June 27, 2024, in Atlanta. (Andrew Harnik/Getty Images)

The trio of top Democrats has made no public indication that they’re pushing Biden to get out of the race, and a White House spokesperson told Fox News on Wednesday after conversations with Schumer and Jeffries, "The president told both leaders he is the nominee of the party, he plans to win, and looks forward to working with both of them to pass his 100 days agenda to help working families."

Meanwhile, there are 20 congressional Democrats publicly calling on Biden to step aside, including Rep. Adam Schiff, D-Calif., a top Pelosi ally.

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Vice President Kamala Harris is increasingly being viewed as a likely successor to the president, despite similarly struggling with her approval numbers. However, some Republicans have privately expressed concerns that a younger candidate could fare better against Trump.

Johnson said he believes that Trump, on the other hand, is seeing a fresh sense of unity and support ever since the attempt on his life last weekend at his rally in Butler, Pennsylvania, where one rally attendee was killed and two others critically injured.

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Vice President Kamala Harris speaks at a campaign rally on June 28, 2024, in Las Vegas. (Bizuayehu Tesfaye/Las Vegas Review-Journal/Tribune News Service via Getty Images)

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"I have done events in 144 cities, in 31 states, they told me, in the last seven months, and there is something happening out there in the country right now. And I think post the failed assassination attempt, there's even a greater energy out there," Johnson said.

"People feel it personally, and they know what President Trump has had to go through. He has a sympathy factor on top of all the energy that was already there. I think we're headed for a really strong November for the party."

Fox News Digital reached out to the Biden campaign for comment but did not hear back by press time.