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.