Despite an influx of campaign cash from small donors in July, Mr. Trump’s operation still largely resembles the bare-bones outfit that he rode to victory during the primary season, more concert tour than presidential campaign, according to interviews and documents filed with the Federal Election Commission through Saturday night. And some Republicans believe he is effectively out of time to invest in the kind of large-scale infrastructure that the Democratic nominee, Hillary Clinton, will bring to the polls in November.

Mr. Trump spends little on polling and made his first advertising purchase of the general election campaign only last week. His rapidly growing digital fund-raising and voter-targeting operation is a partnership with the Republican National Committee, relying significantly on lists built and maintained in recent years by the party…

But it also highlights the bind in which Republican leaders find themselves as Mr. Trump’s struggles threaten to undermine the party’s Senate and House candidates in November: As dependent as Mr. Trump is on their organization, the party is now deeply dependent on Mr. Trump’s surging base of small donors to finance it.

“There is no moving the turnout operation or the absentee ballot program away from Donald Trump and in some senator’s favor,” Reince Priebus, the Republican National Committee chairman, said in an interview on Sunday with CBS’s “Face the Nation.” “It doesn’t work that way. There’s no hundred million dollars in a drawer that might not be spent on one person, but in favor of another.”