“This is one of those moments where the president of the United States has correctly and fully utilized the influence he has as the titular head of the Republican Party,” said Scott Jennings, a Republican strategist who is close to Senator Mitch McConnell, the majority leader. “It’s the hand of Donald Trump, recognizing that we cannot afford to lose Senate seats because we are fighting with ourselves.”

By ending their primaries against the two senators, Mr. McDaniel and Mr. Tarkanian snuffed out what was left of Mr. Bannon’s vision for remaking Congress with candidates in his populist mold. At one time, Mr. Bannon envisioned primaries in Mississippi, Nebraska, Arizona, Wyoming and Nevada.

Now he has none.

With Mr. Bannon exiled by the president and Mr. Trump getting along with congressional leaders and taking counsel from largely mainstream Republicans in the post-Bannon White House, there is little political space for right-wing insurgents to oust incumbents.

Perhaps no one is happier about that than Mr. McConnell, whom Mr. Bannon had once threatened to unseat. Chris Pack, the spokesman for the Senate Leadership Fund, a political action committee that Mr. McConnell helped found to ensure the re-election of incumbents, was succinct on Friday.

“Who is Steve Bannon?” Mr. Pack asked wryly.

Mr. Bannon declined to comment.

Democrats said on Friday that the dynamics of the Nevada race had not changed much, and that they remained optimistic about their chances to pick up a Senate seat in a state won by Hillary Clinton. Mr. Tarkanian, party strategists said, has already caused lasting damage to Mr. Heller by forcing him into positions he will later regret in his campaign against Representative Jacky Rosen, the Democrat whose seat Mr. Tarkanian is now seeking. (Mr. Tarkanian lost to Ms. Rosen by about one percentage point in 2016.)

Last year, for instance, Mr. Heller agonized for weeks over Republicans’ drive to repeal the Affordable Care Act, before finally siding with party leadership and voting to undo President Barack Obama’s signature health care law. Mr. Tarkanian pounced, accusing Mr. Heller of having “no convictions.” And Democrats are still reminding the senator of that vote.

“Nevadans learned today what Senator Heller got for selling out their health care and putting himself and his party first,” Lauren Passalacqua, spokeswoman for the Democratic Senatorial Campaign Committee, said on Friday, adding that “no amount of Trump tweets” could save him.