WASHINGTON — Senate Republicans breathed a sigh of relief in late June. Senator Mitch McConnell’s campaign to lure Senator Marco Rubio into running for re-election in Florida had paid off and Republican control of Congress felt a little more secure.

It has been all downhill since.

Donald J. Trump’s post-convention plummet in the polls has taken vulnerable Republican Senate candidates along for the ride, throwing the door open to the prospect of a Democratic majority under the leadership of Senator Chuck Schumer of New York.

On top of that, the Trump campaign brought on Stephen Bannon, the head of the conservative media outlet Breitbart News, which has savaged Republican congressional leaders like Mr. McConnell and Speaker Paul D. Ryan as hypocritical quislings.

The new campaign team is also promising to let Mr. Trump be Mr. Trump, a prospect that no doubt terrifies many Republican candidates who would prefer not to comment on more incendiary statements from the Republican nominee. His blanket expression of regret Thursday night for earlier comments cheered some in his party, but that moment was quickly overtaken by news of the resignation of Paul Manafort, the embattled Trump campaign chairman.