Bowing to threats posed by the coronavirus, President Trump reversed course on Thursday and canceled the portion of the Republican National Convention to be held in Jacksonville, Fla., just weeks after he moved the event from North Carolina because state officials wanted the party to take health precautions there.

The surprise announcement threw one of the tent-pole moments of Mr. Trump's re-election effort into limbo, with the president describing in vague terms how the Republicans would hold his renomination in North Carolina and do "other things with tele-rallies and online." It was an ill-defined sketch of an August week that Mr. Trump once envisioned drawing huge crowds and energizing his struggling bid for a second term.

While Mr. Trump has spent weeks urging Florida and other states to reopen their economies and return to life as normal, virus cases have surged in Jacksonville and across the region. The president had insisted on moving ahead with the event until Thursday, talking up the big party that Republicans would hold in Jacksonville even with the dangers of large gatherings and some G.O.P. leaders saying they would not attend.