“If Trump is openly breaking with him, that dramatically lowers his capital,” said Dan K. Eberhart, an Arizona oil investor and Republican donor who has spoken to Mr. Bannon about his plans to build an antiestablishment political operation. “He is a strategic thinker, and a lot of the things he said make sense, but this stuff from the book — I’m not going to defend that.”
Mr. Bannon has never been bashful about his ambitions — or his belief in his ability to bounce back. When he was fired from his position as Mr. Trump’s chief strategist in the White House in August amid an uproar over Mr. Trump’s less-than-full condemnation of a white supremacist rally in Charlottesville, Va., Mr. Bannon predicted that he would become even more influential on the outside.
Mr. Bannon proclaimed that he would stage a revolt against Senator Mitch McConnell, Republican of Kentucky and the majority leader, recruiting primary race challengers who could defeat incumbent Republican senators and remove Mr. McConnell from his post. But building and sustaining costly infrastructure is difficult, especially for antiestablishment crusades like the one envisioned by Mr. Bannon, who in a speech at a September rally for Roy S. Moore, the Alabama Senate candidate, warned the Republican establishment and its donors that “your day of reckoning is coming.”
As significant, Mr. Trump, loyal to only a party of one and hungry for legislative success, was never committed to aiding a coup against congressional Republicans. Indeed, Mr. Trump effectively sided with the party establishment by helping to orchestrate his former aide’s political banishment, making Mr. Bannon the fall guy for an embarrassing string of political setbacks, most notably Mr. Moore’s defeat.
His associates said he was planning a political nonprofit group called Citizens of the American Republic that would spend between $25 million and $30 million — though some advisers predicted a budget as large as $100 million — to seed the primary campaigns. Half the budget was slated to be spent on voter mobilization efforts on the ground in targeted districts for months leading up to elections, with the other half going toward advertising, according to people familiar with the plan. While Mr. Bannon publicly bashed the Republican donor class, he spent months quietly courting some of the deepest pockets on the right.
Mr. Bannon discussed his plans in grandiose, but vague, terms in a series of meetings with donors, including the Las Vegas casino magnate Sheldon G. Adelson; a Chicago Cubs owner, Todd Ricketts; and a Home Depot founder, Bernard Marcus.
While Mr. Bannon boasted that he was turning donors against Mr. McConnell, some of the donors who talked to Mr. Bannon say he appeared to lack the organization to effectively raise and spend their money. For instance, they said Mr. Bannon did not have staff follow up to collect donations after meetings — a common step in political fund-raising. Other donors, they said, resented that word of their conversations with Mr. Bannon seemed to leak to the news media in what they regarded as a presumptuous effort to use them to exaggerate his support.
After reports of Mr. Bannon’s meetings with Mr. Adelson, a representative for the Las Vegas billionaire, Andy Abboud, issued a rare statement pledging loyalty to Mr. McConnell and declaring, “The Adelsons will not be supporting Steve Bannon’s efforts.” Mr. Abboud added, “For anyone to infer anything otherwise is wrong.”
The loss of the Mercer family’s support will be particularly hard for Mr. Bannon to overcome. The Mercers began drifting from Mr. Bannon months ago amid concerns about how the controversy he was generating was affecting the family, according to family associates. The Mercers were upset further when they learned that Mr. Bannon had privately boasted that they would back him if he ran for president, according to one family associate.
The family has pumped tens of millions of dollars into businesses and groups that formed the platform from which Mr. Bannon has waged his populist antiestablishment crusade. Besides their share of Breitbart, they are invested in the political data firm Cambridge Analytica, where Mr. Bannon sat on the board, and the investigative nonprofit group Government Accountability Institute, which Mr. Bannon co-founded.
In 2015, the institute produced an investigative book called “Clinton Cash” that damaged Hillary Clinton’s then-nascent presidential campaign. And later that year, the institute quietly assembled a research dossier on Mr. Trump’s ties to organized crime-linked figures in the United States and Russia, which it shopped to multiple news media outlets. At the time, the Mercers were supporting the campaign of Senator Ted Cruz, Republican of Texas. An institute spokesman said that Mr. Bannon had nothing to do with that research, and noted that the group did similar research on other candidates, including Mr. Cruz.
After Mr. Trump vanquished Mr. Cruz, the Mercers threw their support — and $2 million in “super PAC” donations — to Mr. Trump and eventually urged him to hire Mr. Bannon.
But there were repercussions to the Mercers’ support for Mr. Bannon and Breitbart News, which at times allied with white nationalists and other fringe slices of the conservative base. Liberals stepped up pressure last year on institutional investors such as university endowments, foundations and pension funds to withdraw their money from Renaissance Technologies, the $50 billion hedge fund at which Mr. Mercer was a top executive.
Amid that pressure, Mr. Mercer announced his plans to step down from the helm of Renaissance in a November letter that sought to create distance between himself and Mr. Bannon. “I make my own decisions with respect to whom I support politically,” he wrote. “Those decisions do not always align with Mr. Bannon’s.”
Ms. Mercer privately blamed Mr. Bannon for at least part of her father’s predicament and declared that she was permanently severing political ties with Mr. Bannon, according to multiple associates with whom she discussed the matter. Her family cut off funding for Mr. Bannon’s personal protective detail, one associate added. And at the annual holiday party at the Mercers’ Long Island estate, one of the most sought-after tickets for Trump-friendly Republicans, Mr. Bannon was not present.
The Government Accountability Institute distanced itself from Mr. Bannon, who reports suggested was working with the institute on a “Clinton Cash”-style investigation of Mr. McConnell and the political establishment. An institute spokesman stressed that Mr. Bannon has had no affiliation with the institute since Ms. Mercer replaced him as chairman last year when he joined the Trump campaign.
Doug Deason, a major Republican donor and supporter of Mr. Trump, said Mr. Bannon and the Mercers in many ways were a good match, but that Mr. Bannon undermined the potential effect of the partnership.
“They were attracted to him because he’s so brilliant and he’s had some great ideas, but the problem is that he’s got this huge, huge ego — bigger than Donald Trump — and it gets in the way of his success.”
Mr. Deason and Mr. Eberhart both said that the passage of tax overhaul legislation by the Republican Congress made them less likely to support Republican primary challenges like those Mr. Bannon has championed.
Mr. Bannon had a momentary victory on that front in September when Mr. Moore easily won the Republican nomination in Alabama, overcoming the opposition of Mr. Trump and Mr. McConnell. And he crowed when Senators Jeff Flake of Arizona and Bob Corker of Tennessee, both of whom have harshly criticized Mr. Trump, said they would not run again in 2018.
Yet Mr. Moore’s nomination did not inspire a field of insurgents to enter 2018 races, and now it is not clear where Mr. Bannon will be able to claim an electoral scalp. After Mr. Trump’s broadside on Thursday, a number of his preferred candidates found themselves under pressure to denounce Mr. Bannon and some, including former Representative Michael Grimm of New York, did just that.
Another of his candidates, Danny Tarkanian, who is challenging Senator Dean Heller of Nevada, did not rule out eventually appearing with Mr. Bannon but distanced himself from the strategist. “I got in the race before I met or talked to or had any relationship with Steve Bannon,” Mr. Tarkanian said. “I got in the race because of my support for President Trump’s ‘America First’ policies.”
While Mr. Bannon won a wealth of news media coverage for his threats, veteran Republicans were mystified about his strategy of unseating incumbent senators: only eight Republican-held seats are up for grabs in November, hardly the sort of critical mass needed to oust Mr. McConnell.
Just as puzzling was when Mr. Bannon indicated he would target Trump loyalists and reliable conservatives such as Senator John Barrasso of Wyoming, even floating the prospect of Erik Prince, a former Blackwater executive, as a potential challenger in the state.
“If you drilled down into it, it was fantasyland given what the Senate landscape look liked,” said Brian J. Walsh, a Republican strategist, of Mr. Bannon’s attempted insurrection.
And Mr. Bannon effectively limited his ambitions by declaring a truce with most House Republicans, even though his politics are more at odds with those of Speaker Paul D. Ryan than Mr. McConnell, and largely ignoring governor’s races.
Mr. Bannon declined to comment on Thursday.
But late Wednesday night on his radio program, he sounded contrite, betraying none of the defiance and bluster that has landed him on the president’s enemy’s list. He reassured listeners that he stood solidly behind Mr. Trump. “The president of the United States is a great man,” Mr. Bannon told one caller.
An earlier version of this article misstated the middle initial of a Republican strategist who spoke of Stephen K. Bannon’s attempted insurrection. He is Brian J. Walsh, not Brian O. Walsh.