Sen. John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) is defending fellow Arizona Republican 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, who is coming under fire from President Trump.

McCain, who was recently diagnosed with brain cancer, said on Twitter Thursday that Flake "is a principled legislator & always does what's right" for Arizona.

"Our state needs his leadership now more than ever," McCain added.

.@JeffFlake is a principled legislator & always does what's right for the people of #AZ. Our state needs his leadership now more than ever. — John McCain (@SenJohnMcCain) August 17, 2017

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Trump lashed out at Flake, who is up for reelection in 2018, earlier Thursday. The president called Flake weak on immigration and crime, while also praising his primary opponent.

"Great to see that Dr. Kelli Ward is running against Flake Jeff Flake, who is WEAK on borders, crime and a non-factor in Senate. He's toxic!" Trump said.

The back-and-forth comes as Trump is scheduled to hold a rally in Arizona next week, providing an opportunity for him to criticize Flake and McCain in their home state if he chooses.

Flake and Trump have traded rhetorical barbs for years, and the president has reportedly floated the prospect of spending millions to help unseat him.

Ward is a former state senator who mounted an unsuccessful primary challenge against McCain. She's tried to align herself closely with Trump, attacking both GOP senators for being too critical of the president.

The White House has yet to endorse a primary opponent to Flake, but Trump's tweet is a major boost to Ward, who is currently Flake's only challenger for the GOP Senate nomination.

Other candidates could jump in, including state treasurer Jeff DeWitt and former Arizona GOP chairman Robert Graham.

Flake quickly emerged as a prominent GOP skeptic of Trump's presidential campaign and defended McCain against Trump's criticism during the election.

They also clashed last year during a closed-door meeting between then-candidate Trump and the Senate GOP caucus where Trump warned that Flake, who wasn't running for reelection at the time, could lose his seat.