Mr. Trump’s campaign is planning an event at an airport hangar in Portsmouth, N.H. But the state’s governor, Chris Sununu, a Republican, has said he will not be attending. It isn’t clear how many other Republican elected officials will come. The number of attendees could be low, or it could be expansive. There could be lots of people drifting in from Vermont, Maine and Massachusetts.

Campaign officials believe they will be able to prevent the kind of ticket prank that helped turn Mr. Trump’s rally last month in Tulsa, Okla., into a far smaller event than expected — but they still cannot say for sure. And most significantly, there is the looming threat of the coronavirus spreading in a crowd where attendees will be in relatively close quarters, despite being mostly outdoors.

“It’s not what we need right now in terms of Covid,” said Tom Rath, a Republican former New Hampshire attorney general. “We have been very, very fortunate — our number of deaths are quite small.”