WASHINGTON — Arizona Sen. Jeff Flake is leaving Congress because the kind of conservatism he champions no longer seems welcome in Steve Bannon's GOP.

Sen. Jeff Flake, R-Ariz.

Flake, one of the few senators willing to speak up against President Trump, said Tuesday he would not seek re-election. He was facing a tough primary challenge from Kelli Ward, a populist who already had secured the backing of Bannon, Trump's former chief strategist.

“It is clear at this moment that a traditional conservative who believes in limited government and free markets, who is devoted to free trade, who is pro-immigration has a narrower and narrower path to nomination in the Republican Party,” Flake said on the Senate floor Tuesday, in announcing he would not be seeking another term. Later, Flake said to reporters, “if I could run the kind of race I'd like to run and believe I could win a Republican primary, I might go forward.”

Before being elected to the Senate in 2012, Flake had served six terms in the House, where he built a reputation as a crusader against wasteful government spending. He built a reliable conservative voting record  — the American Conservative Union gave him a 93% lifetime rating — but his criticism of the president had alienated many of Arizona’s far-right voters, the very voters most likely to show up for midterm primaries. 

Trump was not timid in punching back at Flake, saying over the summer that he was "weak on borders" and "toxic."

Flake became a key target of Bannon and his band of anti-establishment allies, who have vowed to put up candidates against any senator — in either party — who was hampering Trump’s agenda or was too close to Majority Leader Mitch McConnell of Kentucky.

Bannon, who was a senior adviser to Trump until this summer when he returned to his post as the head of Breitbart News, even flew down to Arizona for a campaign rally for Ward last week.

“Today Steve Bannon and the entire Trump movement added another scalp to his collection and other Never Trump incumbents should be very nervous right now,” Andrew Surabian, a senior adviser for the pro-Trump advocacy organization Great America Alliance and a close ally of Bannon’s, told USA TODAY shortly after the announcement.

“Arizona voters are the big winner in @JeffFlake's decision to not seek reelection. They deserve a strong, conservative in the Senate,” Ward tweeted after the announcement. “Who supports @POTUS & the 'America First' agenda. Our campaign proudly offers an optimistic path forward for Arizona & America #MAGA” she continued.

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Danny Tarkanian, the anti-establishment Republican whom Bannon has indicated support for and who is running against GOP Sen. Dean Heller in Nevada, was invigorated by the news.

“It will be interesting to see whether Heller decides to follow the Corker-Flake model or the Luther Strange model. Either way he will not be the Republican nominee,” Tarkanian told USA TODAY. Strange is a sitting senator from Alabama who was endorsed by GOP leadership and Trump but was defeated last month in the GOP primary by Judge Roy Moore, a controversial candidate endorsed by Bannon.  

When Flake was asked by reporters what his response was to the Bannon wing claiming victory after his announcement, he responded: "Nothing at all."

 Tennessee Sen. Bob Corker,  another lawmaker who is retiring at the end of his term and has battled with Trump, said Flake "is kind of quiet but he stands for all the things that have made our nation great and he’s willing to continue to say that even though we have demagogues and others who end up trying to take us in other directions.”

Flake's departure may just be a signal that some people cannot compete in the new Republican political climate.

“Most incumbents are realizing that the old ways of doing things, aren’t going to work anymore. The old ways of raising money and doing TV ads isn’t going to work,” said Adam Brandon, president of the conservative advocacy group FreedomWorks. Brandon said that while Flake’s voting record had always been good with FreedomWorks, he thought the Arizona senator was “too close to the swamp.”

Contributing: Herbert Jackson and Erin Kelly