In a typical election year, donors whose candidates have dropped out of the race funnel additional contributions to another candidate in the same party.

But this presidential election is different. Of the donors who gave at least $200 to Jeb Bush, Gov. John Kasich, Gov. Chris Christie or Senator Lindsey Graham in the Republican primaries, more of them have also contributed to Mrs. Clinton than to Mr. Trump, according to Federal Election Commission filings through June.

People who give to multiple candidates are a small percentage of Republican donors. Of the donors to Mr. Bush who also gave to one of the current nominees, 303 — more than 2 percent of the total — gave to Mrs. Clinton. Less than 1 percent of them gave money to Mr. Trump, the filings showed.

Those crossover donations are adding up. Mrs. Clinton has received $2.2 million from donors to candidates who dropped out of the Republican presidential primary, about $600,000 more than Mr. Trump has received from such donors, the filings showed.

Donors do not always adhere to their candidate’s lead. Since February, when Mr. Christie endorsed Mr. Trump, 103 of the governor’s donors have given to Mrs. Clinton and 49 have given to Mr. Trump.

Usually, donors to losing candidates in the Republican primary give more to the eventual nominee.

In previous years, donors to Republican candidates who dropped out of the race contributed to the nominee in fairly high percentages. But donors to Mr. Trump’s primary opponents are backing him at a historically low rate.

Only 2 percent, on average, had made a contribution to Mr. Trump’s campaign by July, according to the most recent fund-raising figures the campaigns have filed with the commission. Donors to more moderate Republican establishment candidates gave at the lowest rate, the filings showed.

In 2012, donors who had backed Representative Ron Paul gave at a rate of less than 2 percent to Mitt Romney, the Republican nominee.

Mr. Trump has received money from a smaller proportion of his party’s donors than any candidate since 1980, the filings showed.

In the primaries, he was competing against an unusually large field and started fund-raising much later than most candidates after initially promising to self-fund his campaign.