Campaign aides are hoping they can get their candidate back on the road and back on message, as they try to narrow a gap that they insist is smaller than in the public polls which show him trailing Mr. Biden nationally and in battleground states. But they have just 24 days and one remaining debate to do it.

“Our races have certainly gotten tighter since the presidential debate,” said Sarah Chamberlain, president and chief executive of the Republican Main Street Partnership. She expressed confidence that Republicans running in suburban congressional districts could still succeed, but added “everybody talks about all the days [left], but really in a lot of states, people are already voting.”

Mr. Trump, who has been frustrated by his confinement in the White House, is planning to resume in-person events Saturday, nine days since the announcement he tested positive for Covid-19. He is scheduled to host a gathering at the White House on Saturday and a rally in Florida on Monday. His doctor said in a memo released Thursday that Mr. Trump was ready to resume activities Saturday, but has provided limited details on the president’s condition.