“Biden’s success in the suburbs makes him an acceptable alternative to Trump,” said Scott Reed, the top political adviser for the United States Chamber of Commerce. “His turnout in the suburbs threatens the Republican Senate.”

That presents Mr. Trump with a confounding new political landscape, one that close advisers concede he had seemed unwilling or unable to accept until Wednesday, when he addressed the nation about the pandemic.

“If it was Warren or Bernie and you don’t have coronavirus, I think Trump might sneak by,” said Kevin DeWine, the former chairman of the Ohio Republican Party. “But if it’s Biden, ‘My I.R.A. has tanked,’ and we’re going to have complete disruption because of coronavirus, I think it’s a totally different story.”

Ohio, a traditional presidential battleground that has been trending Republican, would be in play again, Mr. DeWine said. Such Midwestern states were central to Mr. Trump’s election in 2016, and are likely to be similarly crucial this year.