Now, though, his personal indifference toward the virus could threaten his own health, the stability of the country and his already dimming hopes for re-election.

As stock futures fell overnight Friday, strategists in both parties and even senior aides to Mr. Trump said the president would face a harsh judgment from voters for throwing the country into greater uncertainty after one of the most trying years in American history.

“It’s hard to imagine this doesn’t end his hopes of re-election,” said Rob Stutzman, a Republican consultant, pointing to Mr. Trump’s “flaunting of obvious precautions.”…

In the White House, advisers to the president acknowledged that the positive test would remind voters of how dismissive Mr. Trump had been about the virus, not only with his own neglect of safety but also in his overly rosy assessments about a pandemic that has killed more than 207,000 people in the United States. Mr. Trump’s recklessness, one adviser admitted, amounted to a political “disaster.”

It’s almost certain that the remaining two debates between Mr. Trump and Mr. Biden will be canceled or drastically changed.