Sen. Jeff FlakeJeffrey (Jeff) Lane FlakePoll: Trump easily defeats potential GOP rivals in hypothetical primary matchups A primary challenger would weaken Trump in general election, says polling editor Trump 'unlikely' to face primary challenger, says pollster MORE (R-Ariz.) is throwing his weight behind Sen. John McCainJohn Sidney McCainJoy Behar torches Ivanka Trump over emails: 'How dumb' can she be? Stephen King calls Trump a 'chickens---' Trump stokes new unlikely feud MORE (R-Ariz.), endorsing his home state colleague against any potential primary foes.

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"I am," Flake responded when asked if he will back McCain in his upcoming reelection bid.

Flake also predicted that McCain would win.

"He's always prepared for a challenge and I think he'll be just fine, whatever challenge is out there," Flake told The Hill.

The endorsement could help McCain against a Tea Party challenge. Flake remains popular with Arizona's conservative base despite his split from activists on immigration reform.

Both Reps. Matt SalmonMatthew (Matt) James SalmonArizona voters like Kyl but few think he'll stick around Former Sen. Jon Kyl to replace McCain in Senate Arizona governor faces pressure over McCain replacement MORE (R-Ariz.) and David Schweikert (R-Ariz.) have left the door open to a possible run against the longtime senator and former GOP presidential nominee. And the president of the well-funded Club for Growth said Tuesday that the group might back one of them if they decided to run.

McCain didn't have much to say about the Club's comments, and while he refused to say definitively that he'll run for reelection, he told The Hill an announcement is "not too far away."

"These organizations are free to make their own decisions. I have to make mine. And I guarantee you that if I decide to run, and most likely I will, I'll run a tough, strong campaign, and I'm confident we can win," he said.

McCain said he would "most likely" make a decision sometime in the spring and repeated that he's "most likely to run."

Flake's announcement that he'll back McCain may put him at odds with the Club, one of his biggest benefactors in his own primary win in 2012. But he and McCain have worked closely together since Flake won his seat, helping to craft the bipartisan immigration reform bill that passed the Senate last Congress.

This article was updated at 4:35 p.m.