Allies of Senate Majority Leader Mitch McConnell declared open warfare on Wednesday against Stephen K. Bannon, the former White House chief strategist and leader of an insurrection aimed at defeating mainstream Republican candidates in next year’s midterm elections.

More than a year ahead of the 2018 congressional contests, a ­super PAC aligned with McConnell (R-Ky.) revealed plans to attack Bannon personally as it works to protect GOP incumbents facing uphill primary fights. The effort reflects the growing concern of Republican lawmakers over the rise of anti-establishment forces and comes amid escalating frustration over President Trump’s conduct, which has prompted a handful of lawmakers to publicly criticize the president.

Yet the retaliatory crusade does not aim to target Trump, whose popularity remains high among Republican voters. Instead, the McConnell-allied Senate Leadership Fund (SLF) will highlight Bannon’s hard-line populism and attempt to link him to white nationalism to discredit him and the candidates he will support. It will also boost candidates with traditional GOP profiles and excoriate those tied to Bannon, with plans to spend millions and launch a heavy social media presence in some states.

The turbulence presents a danger to Republicans’ narrow 52-seat majority in the Senate, with seasoned GOP lawmakers deciding against seeking reelection amid the political storm — and with many GOP voters cheering the rancor that Bannon has stoked from his perch at his website, Breitbart.

Sen. Jeff Flake (R-Ariz.), in an emotional plea Tuesday, said that he would not run in 2018, after Sen. Bob Corker (R-Tenn.) had done the same in late September. Both men, no longer accountable to Republican primary voters, have taken on higher-profile roles as critics of the president, with Corker calling for a “day care” to step in and control him and Flake calling Trump’s behavior “unacceptable.”

Some Republican lawmakers have privately fretted that simply speaking out against Trump’s incendiary statements or the ­Bannon-aligned candidates that are rousing anger in their states will not be enough — and could backfire — as they try to survive the surge of grievance-driven politics that has gripped the GOP’s base.

“It’s tough,” Flake told CNN on Wednesday. “I’m competitive. I like to fight these battles. But I also knew that I couldn’t run the kind of race that I would be proud of and win in a Republican primary at this time. The politics in that way have changed.”

In the wake of Flake’s announcement, the SLF called Arizona state Sen. Kelli Ward, Bannon’s pick to replace Flake, a “conspiracy theorist” and promised to ensure her defeat.

In recent weeks, Bannon has held court as dozens of candidates have streamed through his Capitol Hill townhouse, including Ward last week, urging them to pledge to vote against McConnell for majority leader. Mississippi state Sen. Chris McDaniel (R) is close to making a decision on a bid against Sen. Roger Wicker (R-Miss.) and has won Bannon’s blessing, according to a person close to him.

Hedge fund executive Robert Mercer and his daughter Rebekah — Bannon’s wealthy allies — have pledged millions to the cause, said people briefed on their plans.

Bannon’s critics argue that he is causing unnecessary internal divisions that could make it harder to pass tax legislation — and to win general elections next fall. They also point to Sen. Luther Strange’s defeat in last month’s Republican primary in a special Senate election in Alabama as an example of a dynamic they worry could repeat itself across the next year if left unchecked. The SLF spent more than $10 million to help Strange.

What Flake's retirement means for the Senate in 2018

Strange was endorsed by Trump and McConnell but lost to a former state judge, Roy Moore, who had won the backing of Bannon and his orbit of allies. Moore is now in a tight race with Democratic nominee Doug Jones.

“This is a guy who is more interested in seeing his name in the headlines than he is in any kind of accomplishment for the president,” said Josh Holmes, a former chief of staff to McConnell who has been increasingly outspoken in his opposition to Bannon. “It comes at great cost to the president’s term, the Republican Party and any hope they can have at working as a team.”

On Wednesday, the SLF’s Twitter account mocked Danny Tarkanian — a frequent conservative candidate in Nevada who is challenging Sen. Dean Heller (R-Nev.) — for suggesting that Heller join him in pledging to oppose McConnell as majority leader.

The tweets also turned the spotlight on Bannon.

The SLF tweeted a 2016 headline from the New York Daily News — “Anti-Semitic Trump campaign CEO Stephen Bannon not a big fan of ‘whiny brat’ Jews, ex-wife says” — with space for Tarkanian’s signature.

“Here’s another pledge for @DannyTarkanian to sign,” the PAC tweeted.

The Daily News reported last year that in a 2007 court statement, Bannon’s former wife Mary Louise Piccard said he didn’t want their twin daughters attending a school because too many Jews attended. “The biggest problem he had with Archer [School for Girls in Los Angeles] is the number of Jews that attend,” Piccard said in her statement, the newspaper reported.

Bannon’s spokeswoman Alexandra Preate told The Washington Post last year that Bannon has denied saying that and proudly sent the girls to Archer. And after seeing the SLF tweet to Tarkanian on Wednesday, a Bannon confidant said, he responded to the group’s social media blitz with laughter — as he has to other recent attacks on his character.

That attack was one of a number of swipes at Bannon that have popped up as the former White House adviser has emerged as a player on the national political scene. Holmes and others have called him a “white supremacist.”

Holmes defended his use of the term to describe Bannon in a recent interview. “If you look at his associations, the people who are ecstatic about his efforts, the long history of conduct that he has been a part of, I will let others come to their conclusions,” he said. “But this is not a guy that just burst on the scene. Let’s not act like everybody doesn’t know exactly who Steve Bannon is.”

Bannon has said repeatedly that his nationalist, populist vision includes a place for all races, sexual orientations and genders to succeed.

“Do we need any further evidence than Mitch McConnell and his cronies reducing themselves to using left-wing talking points to attack Steve? It’s pathetic to watch,” said Andy Surabian, a senior adviser to the Great America Alliance super PAC and Bannon’s former deputy at the White House. The pro-Trump PAC is engaged in a slew of races, sending around a bus to rally activists, and counts longtime operative Edward Rollins as its strategist. “Every poll shows Mitch McConnell is an albatross on the Republican candidates,” Surabian said. “If McConnell truly cared about our Republican majority in the Senate more than he cares about his own power, then he would step down as Senate majority leader today.”

Bannon’s circle says the contents of his divorce proceedings, along with claims of racism, are unlikely to do new damage to his reputation, since he has been targeted in the past along those same lines by Democrats, and even in a “Saturday Night Live” caricature as an angel of death. Advisers also note that the attacks elevate Bannon’s profile, which could help carry his anti-establishment message.

In an email, SLF President Steven Law said he expects Bannon to become a liability for any insurgent candidates he supports in 2018.

“But his real impact would be felt in general elections, where Bannon’s well-documented, toxic views and alt-right paper trail could become a liability for candidates who are perceived as closely tied to him,” Law said.

According to public polling, neither McConnell nor Bannon is in good standing with voters. In an April poll conducted by Quinnipiac University, one of few to ask voters about Bannon, just 11 percent said they viewed him positively and 45 percent said they viewed him negatively. The numbers were better among Republicans, with a nine-point favorability margin.

Sen. John Barrasso (R-Wyo.), one of the incumbents mentioned as a possible target for Bannon in 2018, dismissed the effort to get Senate Republicans to sign pledges vowing to oppose McConnell.

“No, I haven’t signed pledges. I didn’t sign Grover Norquist’s pledge,” Barrasso said on Wednesday, referring to the anti-tax advocate’s famous pledge. “I’m not a guy that signs pledges, and I’m going to vote for whoever I believe is in the best interests of the people of Wyoming.”

The reasons for Flake’s decision may be tied less to the national environment surrounding the feud between McConnell and Bannon and more about raw home-state politics. Reams of public and private polling in recent months have found Flake to be an unpopular incumbent, especially among the Republican voters he would need to win over in a primary fight next year.

McConnell’s polling among Republicans has tumbled since the start of the year — a fact that Bannon has used to his advantage. On Tuesday, Bannon’s Breitbart website reported that a Harvard-Harris poll found 56 percent of Republicans in favor of dumping McConnell as leader. That same day, at a media roundtable organized by the conservative Heritage Foundation, a reporter for Breitbart asked members of the House Freedom Caucus to respond to the poll number.

“It was that high?” scoffed Rep. Matt Gaetz (R-Fla.), wondering jokingly about why it wasn’t even higher.

As the two sides spar, Senate Republicans are trying to demonstrate that the party’s agenda is moving forward despite the infighting — and that relations with the White House are smooth. On Wednesday, a day after Flake made his charged speech, Ivanka Trump appeared with Republican senators to pitch a child tax credit that she has lobbied to include in the upcoming tax legislation.

Still, GOP donors who are friendly with McConnell welcomed the effort to thwart Bannon’s attempts to shape the 2018 contests and rattle McConnell as Senate Republicans are trying to work with Trump.

“Absolutely. Why not?” said Al Hoffman, a major Republican donor and McConnell ally. “You’ve got to get rid of the Bannon banner.”

Ed O’Keefe and Paul Kane in Washington and Sean Sullivan in Alabama contributed to this report.