Groups supporting a Republican-held Congress signaled moves to have some of their candidates run as an explicit “check and balance” against a Clinton presidency, an acknowledgment from those groups that the race for the White House may be all but over.

Mr. Trump seemed aware of that Sunday evening as he campaigned in Florida, where he owns properties but where polls also show him behind Mrs. Clinton. Unlike his recent and explicit vows of retaliation against congressional Republicans who have opposed him, Mr. Trump suggested that he needed Republican majorities to enact his agenda.

The Trump camp conceded that it was lagging, but the campaign manager, Kellyanne Conway, said it still saw a path to victory. Mr. Trump’s schedule this week will take him to battleground states like Ohio. But he is also visiting places like Colorado, which few strategists believe he has a chance of winning.

Mrs. Clinton faces an electoral map that is more in flux than Democrats had imagined even a few months ago. The website Real Clear Politics listed Texas, a reliably Republican-leaning state in presidential years, as a tossup. And polls show that an independent candidate, Evan McMullin, is competitive in Utah.