Sen. Susan Collins Susan Margaret CollinsMcConnell locks down key GOP votes in Supreme Court fight Most Americans think winner of election should pick next Supreme Court justice: poll Murkowski: Supreme Court nominee should not be taken up before election MORE (R-Maine) on Monday said she’s unsure whether President Trump will be the Republican Party’s presidential nominee in 2020.

In an interview with MSNBC’s Hallie Jackson, Collins, who wrote in Speaker Paul Ryan Paul Davis RyanKenosha will be a good bellwether in 2020 At indoor rally, Pence says election runs through Wisconsin Juan Williams: Breaking down the debates MORE’s (R-Wis.) name for president in 2016, said it’s “too early to tell” whether Trump will seek a second term.

Asked if she believes Trump will be the party’s 2020 nominee, Collins responded: “It’s too difficult to say.”

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Trump’s campaign has not taken a break since the president’s surprise electoral victory over Democrat Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE in November.

The campaign has been relentlessly fundraising and releasing campaign ads, while the president has taken breaks from his White House duties to hit the road for rallies in the states that he won in 2016.

It is exceedingly rare for an incumbent president to receive a primary challenge. It last happened in 1992, when conservative firebrand Pat Buchanan challenged former President George H.W. Bush.

But Trump’s historically low approval rating and the shadow of the Russia investigation have added to the sense that he could attract a primary challenger or decline to seek a second term.

Trump’s critics within the GOP, including Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (Ariz.), Sen. Ben Sasse Benjamin (Ben) Eric SasseChamber of Commerce endorses McSally for reelection Ben Sasse is mistaken with idea for the election of senators in America Big Ten football to return in October MORE (Neb.) and Ohio Gov. John Kasich, are among those generating early buzz.

But Trump has a solid grip on his base of supporters, and many Republicans think a primary challenge against him would be a suicide mission unless there is movement to impeach him or he decides to walk away.