Sen. Jeff FlakeJeffrey (Jeff) Lane FlakeThe Hill's Morning Report — Trump readies for Putin summit: 'He’s not my enemy’ Overnight Defense: Fallout from tense NATO summit | Senators push to block ZTE deal in defense bill | Blackwater founder makes new pitch for mercenaries to run Afghan war On The Money — Sponsored by Prudential — Lawmakers demand answers from Mnuchin on tariffs | Fed chief lays out stakes of Trump trade war | Consumer prices rise at highest rate in six years | Feds to appeal AT&T merger ruling MORE (R-Ariz.) is throwing his weight behind Sen. John McCainJohn Sidney McCainSenate Dems tell Trump: Don't meet with Putin one-on-one McConnell: Senate to confirm Kavanaugh by Oct. 1 Overnight Defense: Fears rise over Trump-Putin summit | McCain presses Trump to hold Putin 'accountable' for hacking | Pentagon does damage control after NATO meet 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 GOP tinkers with election rules with an eye on McCain's seat Quiet jockeying for McCain seat angers Republicans McSally tells GOP colleagues she'll run for Arizona Senate 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.