Republicans increasingly worry that their base’s contempt for Mr. McConnell is more potent than its love for Mr. Trump. Mr. McConnell could be an anchor around incumbents in the same fashion as Representative Nancy Pelosi, the House Democratic leader, who is routinely used to undermine Democratic candidates. The loudest applause Mr. Moore received during an election-eve rally came when he declared, “Mitch McConnell needs to be replaced.”
In a memo about the Alabama election that circulated among Republican donors, Steven Law, president of the Senate Leadership Fund, a “super PAC” closely allied with Mr. McConnell, said primary voters were intensely angry and inclined to blame Republicans for dysfunction in Washington.
“The Republican Congress has replaced President Obama as the bogeyman for conservative G.O.P. primary voters,” Mr. Law wrote, cautioning that the president was helping to amplify that point of view: “This narrative is driven by Trump himself, and it resonates with primary voters who believe the Republican Congress ‘isn’t doing enough’ (as we frequently heard in focus groups) to advance the president’s agenda.”
Mr. Law, whose group spent more than $10 million to prop up Mr. Strange, said in the memo that Republicans had been damaged by “the Obamacare repeal fiasco,” and said they should expect to fight hard-right primary candidates in Mississippi and Nevada, among other states. Mr. Law derided Mr. Bannon for being focused mainly on “promoting his own brand,” and discounted him as a major force in Alabama.
The convulsive mood on the right has considerably reshaped the political map for 2018, making a favorable list of Senate races somewhat less hospitable to Republicans. Two Republican senators, Dean Heller of Nevada and Jeff Flake of Arizona, have seen their poll numbers collapse after clashing with Mr. Trump and embracing unpopular legislation to repeal the Affordable Care Act.
In Tennessee, Senator Bob Corker, a well-liked lawmaker from a traditional Republican mold, on Tuesday became the first Senate Republican to announce that he would not seek re-election in 2018. His departure is likely to yield a contentious Republican primary, much like the one just concluded in Alabama.
The Alabama race “is going to inspire a lot of people,” Mr. Bannon said in an interview in Montgomery on Tuesday night.
Mr. Bannon said he had held discussions about the Tennessee race with Mark E. Green, a state senator who was nominated to be Mr. Trump’s Army secretary before withdrawing after facing scrutiny for his past statements about gay and transgender people. Tennessee could be the site of the next major populist-versus-establishment conflagration if Gov. Bill Haslam responds to entreaties to enter the race.
Mr. Bannon also said he aimed to oust Mr. Heller, Mr. Flake and Senator Roger Wicker of Mississippi. Ed Martin, a former chairman of the Missouri Republican Party, said Mr. Bannon had also inquired about the state’s Senate race, in which the Republican establishment has rallied around Josh Hawley, the state attorney general, as an opponent for Senator Claire McCaskill, a Democrat.
After leaving the White House last month, Mr. Bannon returned to his perch at Breitbart News, and has been using the hard-right website and his close ties to the Mercer family, New York-based conservative donors, to create a new, insurgent power base.
It remains unlikely that Republicans will lose control of the Senate next year, because the playing field of races is tilted so strongly in their direction. The party is defending just eight seats, mostly in strongly conservative states, compared with 25 seats held by Democrats or independents who caucus with them.
Yet the pitfalls Republicans have encountered so far have created unexpected opportunities for Democrats, and the party is assessing even long-shot races where there is the possibility of an upset. In Tennessee, a solidly Republican state, several new Democrats are considering the race for Mr. Corker’s seat: Mayor Andy Berke of Chattanooga said in a statement that he would explore a bid “in the coming weeks,” and State Senator Jeff Yarbro, a Nashville legislator, is also eyeing the race. One Democrat, James Mackler, a lawyer and Iraq war veteran, is already running.
And in conservative Alabama, both Democrats and Republicans believe Mr. Moore’s nomination may put the seat at risk in a Dec. 12 general election, when he faces Doug Jones, a former federal prosecutor who is the Democratic nominee.
Mr. Jones is scheduled to campaign with former Vice President Joseph R. Biden Jr. next week, and said in an interview on Tuesday that he would seek support from Republican and independent voters who may be repelled by Mr. Moore, who was removed from the bench for defying Supreme Court rulings and has called in the past for banning homosexuality.
In a sign of Mr. Moore’s vulnerability, Mr. Law’s memo described him as off-putting to “business-oriented Republicans,” who “recoil at Moore’s grandstanding.”
It is not only Republican senators who could find themselves cast out by conservative challengers next year. A parade of candidates, aligning themselves explicitly with Mr. Trump, is lining up to take on House Republicans whom they view as insufficiently loyal to the president. If enough Republican lawmakers are ousted in primaries, or forced to spend millions just to secure renomination, it could give Democrats a better chance to pick up the two dozen seats they need to take a majority.
“I think incumbents are extremely vulnerable,” said Barry Moore, an Alabama state representative challenging Representative Martha Roby, a Republican who called on Mr. Trump to withdraw from the presidential race late last fall. “The American people are sending a message that there’s nothing getting done in D.C., and we’re going to have to replace a lot of those people.”
A spokesman for Ms. Roby, Todd Stacy, noted that she and “her House colleagues have voted to repeal Obamacare, roll back Obama regulations, repeal Dodd-Frank, fund border wall construction, rebuild the military, reform the V.A. and tax reform got rolled out today.”
Still, the alarm is most acute in the Senate. Party strategists have seen private polling in a number of states that shows Mr. McConnell deeply unpopular with his fellow Republicans. In Arizona they have found Mr. Flake trailing his primary challenger, Kelli Ward, a former state senator, by a significant margin.
This month, Senator John McCain, Mr. Flake’s Arizona colleague, staged something of a gentle intervention, urging Mr. Flake to move more aggressively to repair his standing in the party, according to two Republicans briefed on the conversation.
Mr. Bannon taunted Mr. Flake on Tuesday night, suggesting that if the Arizona senator “doesn’t get a better poll in the next 30 days, you’re going to see him step down or the establishment is going to make him” — a possibility Mr. Flake’s campaign spokesman discounted.
“He is running,” said Will Allison, a spokesman for Mr. Flake.
In Mississippi, Mr. Wicker, a former chairman of the National Republican Senatorial Committee, has been gearing up early in anticipation of a revolt on the right, hiring a veteran campaign manager, Justin Brasell, and branding himself on social media as a fierce ally of Mr. Trump.
But Chris McDaniel, a Mississippi state senator who nearly toppled Mr. Wicker’s colleague, Thad Cochran, in a 2014 primary, said that Mr. Moore’s victory made a challenge against Mr. Wicker “more compelling” and that he would decide by the end of October. He said he had spoken multiple times with the Mercers in recent months and had received assurances of support.
Mr. Wicker, he charged, had become “Mitch McConnell’s yes-man.”