The Republican-aligned groups with the deepest pockets, the American Crossroads operation co-founded by the Republican strategist Karl Rove and the constellation of political groups overseen by the billionaire industrialists Charles G. and David H. Koch, have abandoned the presidential race, choosing to focus their efforts down the ballot. Republican strategists overseeing the party’s most competitive Senate races have begun crafting their own independent turnout plans, mindful that their candidates need to reach middle-of-the-road voters Mr. Trump has forsaken.

The parties cannot raise unlimited “soft money” contributions — money raised into state rather than federal accounts and parlayed into large-scale political advertising campaigns — as they could during the 1990s, before super PACs became legal.

“In 1996, national party committees could raise both federal money and nonfederal money,” said Benjamin L. Ginsberg, a former chief counsel at the Republican National Committee and a longtime Republican election lawyer. He added, “The 1996 R.N.C. raised significant money on its own it could spend as it saw fit.”

Some Republicans believe that with the fall campaign weeks away, the party should focus its money and efforts down ballot to protect Republicans’ congressional base. That would mean quietly ignoring Mr. Trump’s call this month for a 50-state field operation and instead emphasizing congressional districts and swing states that are also Senate and House battlegrounds.

“They can’t do anything publicly — you can’t rebuke your nominee,” said Liam P. Donovan, a former aide to the National Republican Senatorial Committee. “But you could allocate resources to places where it helps up and down the ballot.”

The difficulty, though, is that as November approaches, the Republican National Committee is more reliant on Mr. Trump for cash than on other recent nominees. Millions of dollars are coming in through a small-donor-focused committee operated jointly with the committee, which is splitting a share of the proceeds with Mr. Trump. Over half the money raised by the Trump campaign and the committee combined in July came from donors giving less than $200, far more than for any recent Republican nominee. (That figure does not include additional small donations raised by a joint fund-raising committee that Mr. Trump’s campaign treasurer controls, which is not required to file disclosures until October.)

The campaign’s large-dollar fund-raising, also run jointly with the Republican National Committee, yielded $16 million in cash in July, but much of it is reserved under law for party accounts dedicated to the Republican National Convention, legal expenses and expenditures on offices. Despite being able to collect far more from each of the biggest Republican donors under new campaign finance rules passed in 2014, the committee is well off pace from its July fund-raising in 2012, when Mitt Romney was the party’s nominee.