Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeFlake to try to force vote on DACA stopgap plan Congress punts fight over Dreamers to March Outgoing GOP rep: Republican Party 'heading into trouble' in election MORE (R-Ariz.) is throwing his weight behind Sen. John McCainJohn Sidney McCainLawmakers worry about rise of fake video technology Democrats put Dreamers and their party in danger by playing hardball Trump set a good defense budget, but here is how to make it better MORE (R-Ariz.), endorsing his home state colleague against any potential primary foes.

"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 SalmonQuiet jockeying for McCain seat angers Republicans McSally tells GOP colleagues she'll run for Arizona Senate GOP Senate hopeful Kelli Ward leads challengers in internal poll 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.