Basically, if you were for Mr. Trump before, you are probably still with him now. If you started out this year generally against him but willing to see how he led during the crisis, you have probably given up on that by now.

Still, it matters that Mr. Trump has proved to have a remarkably high floor. With the Republican establishment and the conservative news media firmly behind him, the president is unlikely to fall far below 40 percent approval. To finish strongly in November, he would need to win back between five and 10 points nationally, relying heavily on persuadable voters.

“A lot of it has to do with where people will be economically going into the election, and who they will credit or blame for that,” Thomas Sutton, a professor of political science at Baldwin Wallace University in Ohio who runs the university’s statewide poll, said in an interview. “At this point in October, the unemployment rate will be less, Trump will claim credit for that, and Biden’s got to run a really strong campaign with a countermessage to stop him from getting that credit and taking it to victory in Ohio.”