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 (R-Ariz.) called for a Republican to challenge President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE in the 2020 election on Sunday during an interview with NBC's Chuck Todd on "Meet the Press."

"Do you think he needs to be challenged from somebody who espouses your views?" Todd asked Flake.

"Yes, I do. I do. I mean, it would be a tough go in a Republican primary. The Republican Party is the Trump party right now. But that's not to say it will stay that way," Flake said.

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The retiring Arizona has long been one of Trump's chief critics in the Republican Party, often lashing out at the president for his immigration and economic policies, as well as for his bombastic rhetoric. He criticized the Trump administration when announcing he would not run again for Senate.

Flake made similar comments earlier this month, and would not rule out a 2020 challenge to Trump.

“I do think the president will have a challenge from the Republican Party, I think there should be,” Flake said. “I also think that there will be an independent challenge, particularly if the Democrats insist on putting somebody up from the far left of the party.”

Flake has stoked speculation about a potential 2020 bid with an upcoming visit to New Hampshire.

Meanwhile, Trump last night jumped into full campaign mode at a rally in Pennsylvania for GOP congressional candidate Rick Saccone.

Trump spent most of the event touting his accomplishments as president, and revealing his new campaign slogan "Keep America Great."

"The tax bill was so massive, bigger than [Ronald] Reagan, biggest one done," Trump said. It is so big, he added, that "we are calling it tax reform" instead of a tax cut.

"This is where a nonpolitician is good. For 40 years they couldn't pass anything," the president continued.

Trump is poised to face off against one of several Democratic hopefuls in 2020, which may include Sens. Elizabeth Warren Elizabeth WarrenGOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (D-Mass.), Kamala Harris Kamala HarrisSocial Security and Medicare are on the ballot this November Harris honors Ginsburg, visits Supreme Court The Hill's Campaign Report: Trump and Biden vie for Minnesota | Early voting begins in four states | Blue state GOP governors back Susan Collins MORE (D-Calif.) or Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.).