MIAMI — Florida billed itself as a coronavirus oasis when it wooed the Republican National Convention: The pandemic seemed under control, and Republican state and city leaders welcomed a major televised event to show off the progress and cement votes for President Trump in the nation’s biggest presidential battleground.

That pitch collided with the grim reality of the coronavirus and its explosive surge in Florida when Jacksonville, the convention’s new host, imposed a requirement on Monday that people wear masks indoors, precisely the mandate that the Republican Party had hoped to avoid for its celebrations.

The mask order is almost certain to rankle Mr. Trump, whose demands for a traditional, packed rally forced the move earlier this month to Jacksonville from Charlotte, N.C., in the first place.

In a must-win state for Mr. Trump — and arguably a must-win city for Republicans — recent polls have shown that Jacksonville residents do not want the convention.

The challenge of confronting the virus and Jacksonville’s racist history while a national political convention comes to town has troubled the city’s business elite from the start, said Nate Monroe, metro columnist for The Florida Times-Union newspaper, with the crush of new infections only intensifying those concerns.