“Right now, the Moore-Bannon faction prevails,” Mr. Shelby said.
Republican leaders on Capitol Hill are making no attempt to mask their fear, predicting that failure to pass a tax overhaul in the coming months will lead to a wipeout in next year’s midterm elections. For the first time, some senators are contemplating whether their advantages on the electoral map next year could crumble amid a wave of primary challenges and other departures, putting their two-seat majority in jeopardy next year.
Republicans are increasingly mystified by their own grass roots, an electorate they thought they knew, and distressed that a wave of turnover in their ranks could fundamentally change the character of Congress. They fear that the inchoate populism that Mr. Trump personifies, and which Mr. Bannon is attempting to weaponize against incumbents, is on the march.
Mr. Trump is not helping. Speaking at a high-dollar fund-raiser for his re-election at Le Cirque restaurant in New York last week, Mr. Trump asked contributors what they would think if he worked with Democrats on health care, should Republicans prove unable to repeal the Affordable Care Act, according to a dinner attendee.
“I might very well end up making a deal with the Democrats,” he said, drawing applause.
“I got to my desk,” he said, recalling his expectations when he arrived in the White House. “I said, ‘Where’s the health care bill, where is repeal and replace?’ And it’s just been constant fighting.”
With Mr. Trump so willing to criticize, his aides have been emboldened to fan the discontent. That was jarringly illustrated this week when Politico revealed that Mr. Ayers told another group of party donors that they should “purge” disloyal Republicans.
“If I were you, I would not only stop donating, I would form a coalition of all the other major donors, and just say two things,” he said. “We’re definitely not giving to you, No. 1. And No. 2, if you don’t have this done by Dec. 31, we’re going out, we’re recruiting opponents, we’re maxing out to their campaigns, and we’re funding ‘super PACs’ to defeat all of you.”
To the consternation of Republican lawmakers, Mr. Pence let Mr. Ayers’s comments stand.
“That was amateur hour,” said Mr. Corker, adding that it also happened to be “the general modus operandi” of the administration.
To maintain their grip on the party, Republican leaders and their supporters are scrambling to recruit new allies and pleading with their loyalists to remain in Washington — not always with success.
A battery of Republican senators had telephoned Tennessee’s governor, Mr. Haslam, a popular and wealthy establishment figure, and invoked Mr. Moore’s primary election victory to argue that the identity of the party was on the line, according to a Republican official familiar with the line of argument. Mr. Corker and Senator Lamar Alexander of Tennessee led the charge.
On Thursday, he turned them down.
In a sign of their determination to retain the seat, establishment-aligned Tennessee Republicans immediately began trumpeting R. Brad Martin, a former state legislator and onetime chief executive of Saks as a potential candidate. Representative Marsha Blackburn, a conservative, could prove the most formidable candidate, though.
And matters could get worse. Senator Susan Collins, Republican of Maine, has said she will announce next week whether she will make a 2018 bid for governor. If she leaves Washington, the decision would be tectonic. The Senate would lose a dealmaker, and her seat could eventually slip to the Democrats.
“A lot of my colleagues have urged me to stay,” said Ms. Collins, adding that she would talk with her family before announcing her decision in Maine.
That would only add to the headaches. Senator Jeff Flake of Arizona, who has taken aim at Mr. Trump, appears so weakened with Republican voters that there is increasingly talk of contingency planning should he not run again or seek re-election as an independent.
If he remains in the race, Mr. Flake, who currently faces one opponent on the right, appears likely to draw additional challengers.
A wealthy Arizona lawyer, Jay Heiler, said in an interview that he is considering a run against Mr. Flake. Mr. Heiler, a close ally of former Gov. Jan Brewer of Arizona, an immigration hawk, said the state should have a senator who gives full support to Mr. Trump.
“The president’s agenda is one which I wholeheartedly endorse,” Mr. Heiler said. “I have not seen the president advance anything which I don’t think is in the best interest of the country.”
Mr. Flake has already drawn a formidable Democratic opponent in Representative Kyrsten Sinema.
What Republicans are even more loath to talk about than Mr. Flake’s fate is the health of two long-serving senators: John McCain of Arizona and Thad Cochran of Mississippi.
Their absence from the Senate for health reasons could make it more difficult for the party to gather the 50 votes needed to pass a tax package. And some top party figures are concerned that if Mr. McCain, who has an aggressive form of brain cancer, has to resign or dies in office, it could hand Democrats an opening to win two Senate seats in one state next year.
That and the uncertain prospects of Senator Dean Heller of Nevada, who is facing a Trump-inspired primary challenger from the right, could endanger the Senate majority or at least offset any gains Republicans make in the conservative states where Democrats are facing re-election next year.
Mr. Cochran’s condition is more shrouded in mystery. He has been ill since returning from official travel overseas this summer and in recent weeks has remained in Mississippi, missing votes. His office issued a statement late last month revealing only that he had “urological issues” and would return to the Senate on Oct. 16.
But Mr. Cochran, who turns 80 in December, is not the only Mississippi senator weighing on the minds of Republicans. Senator Roger Wicker is facing a potential primary next year against a Bannon-allied insurgent, State Senator Chris McDaniel. Republicans are preparing a show of muscular support for Mr. Wicker: The state’s conservative governor, Phil Bryant, has told Republicans that he would back Mr. Wicker forcefully in a primary fight.
Then there is Utah, where the Senate’s longest-serving Republican, Orrin G. Hatch, 83, is still determining whether to run for re-election next year. Republicans are inquiring about Mitt Romney’s willingness to run for the seat, and he has been open to overtures from national donors and his own former campaign aides, according to two people who have spoken with him in recent weeks.
But Mr. Bannon will not silently let Republicans anoint Mr. Romney and has told associates he will mount a campaign to at least bloody the former Republican presidential nominee.
All of that indicates that 2018 could bring back the ferocious infighting of the Tea Party insurgency.
“We had a pretty rough patch back in 2010 and 2012, and just when that sort of calms down, here comes the next wave,” said Senator John Cornyn of Texas, the second-ranking Senate Republican. At least then, he added, the fury was more coherent: “It was more ideological, whereas now it’s just more everybody is just angry at everybody.”
Art Pope, an influential Republican donor in North Carolina, said he and other financial benefactors were growing impatient, a frustration borne out in the party’s dwindling fund-raising. The pressure is mounting for Republicans to secure real victories, he said, with tax reform now the overwhelming goal. And he echoed some newer Republican senators in saying he would favor changing Senate rules to make that more achievable by eliminating the legislative filibuster.
“The Republican Party needs to get legislation done,” Mr. Pope said. “Tax reform is at the forefront of everyone’s list right now. It’s disappointing that health care reform has not passed.”
Asked if the White House or congressional leaders had reached out to reassure him that things were under control, Mr. Pope shot back: “No. It is not under control.”