But the timing of Mr. Trump’s decision on Thursday night was influenced by the imminent need for the Republican Party to book an enormous number of hotel rooms in Jacksonville and sign other costly service agreements, according to multiple Republicans familiar with the plans. The looming deadline for attendees to make deposits on hotel rooms was next Monday, according to a copy of instructions mailed to delegates.

If party officials proceeded to make those reservations and cancel them later, people familiar with the planning said, it would have added financial losses to an already expensive fiasco involving two cities — Jacksonville and Charlotte, N.C., where the convention was originally set — and two sets of donors raising money in both places.

There were other urgent factors involved in the decision, including the health of party officials and delegates coming from across the country, and Mr. Trump’s sinking political standing, which was largely attributable to months of inattentiveness to the virus. It had become increasingly apparent in recent days that Jacksonville organizers were facing open resistance from local officials and that the city’s Republican mayor, Lenny Curry, once a champion of the convention, was too tied up with virus concerns to cheerlead much for it.