WASHINGTON — Wealthy Republican donors, viewing Donald J. Trump as a losing cause, are turning their firepower toward several key Senate races, threatening to bury Democrats under a deluge of negative advertising.

In North Carolina, the One Nation advocacy group just poured $1 million into an ad buy in support of Senator Richard M. Burr, a Republican who has been buffeted by Mr. Trump’s difficulties in the state. That was on top of a $1.5 million infusion weeks before.

The group, which does not disclose its donors, is run by allies of Senator Mitch McConnell of Kentucky, the majority leader, and Karl Rove. It has already spent at least $25 million on other close Senate races, including those in Pennsylvania and New Hampshire.

The Freedom Partners Action Fund, a “super PAC” aligned with Charles G. and David H. Koch, has spent at least $15 million on TV ads, a quarter of it in Pennsylvania, where Senator Patrick J. Toomey is in a tough re-election fight, at least in part because of voters’ distaste for Mr. Trump.