With Donald J. Trump facing a potential rout at the hands of Hillary Clinton, a river of cash from some of the Republican Party’s biggest donors has begun to flow down to Senate and House races in the final days of the 2016 campaign.

Disclosures filed with the Federal Election Commission on Thursday revealed tens of millions of dollars in late donations and transfers to Republican “super PACs” focused on down-ballot races, suggesting a significant last-ditch effort to protect Senate and House candidates against Mrs. Clinton’s surge. Relatively little new money has come into outside groups supporting Mr. Trump.

And Mr. Trump appears to have lost the support of his biggest donor: himself.

Mr. Trump contributed no cash and just $31,000 worth of rent and staff salaries to his campaign in the first three weeks of October, a fraction of the $2-million-a-month self-funding pace he had set since winning the Republican presidential nomination.

As of mid-October, according to the disclosure, Mrs. Clinton was raising $2.8 million each day and had $62 million in her campaign account, four times as much as Mr. Trump.