Sen. Jeff FlakeJeff FlakeThe more complex the tax code, the more the wealthy benefit McCain returning to Senate in time for health vote Trump slams 'sad' Republicans who won't 'protect' him MORE (R-Ariz.) is throwing his weight behind Sen. John McCainJohn McCainMcCain returns to applause, casts deciding vote to advance healthcare bill Senate votes to begin ObamaCare repeal debate The Hill's 12:30 Report 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 SalmonMatt SalmonComey fallout weighs on the GOP Conservative activists want action from Trump Senators fear fallout of nuclear option 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.