But behind the scenes, Republicans are looking at possible contingency plans, including limiting the number of people who descend on Charlotte to only delegates, and making alternate delegates stay home, according to interviews with a half-dozen Republicans close to the planning.

Mr. Trump, who was heavily involved in the staging of his last nominating convention, has even shown a new openness to participating in a scaled-down event. He has mused aloud to several aides about why the convention can’t simply be held in a hotel ballroom in Florida, given all of the health concerns and the fact that Florida is further along in reopening portions of the state.

But with expensive contracts signed and money raised, the event is unlikely to move from Charlotte — unless the Democratic mayor and governor there say it can’t be held.