Tens of millions of dollars have already been spent in a city that will now host little more than a G.O.P. business meeting, and donors are wary of opening their wallets again to bankroll a Jacksonville gathering thrown into uncertainty by a surge in coronavirus cases.

Organizers are trying to assuage vexed Republicans who collectively gave millions of dollars for a Charlotte event that has mostly been scrapped. The host committee there has spent virtually all of the $38 million it raised before the convention was moved, leaving almost nothing to return to donors, or to pass on to the new host city.

In Jacksonville, fund-raisers are describing the process as the most difficult they have ever confronted: Florida has been setting daily records for new virus cases, freezing money as donors wait and worry about the safety risks of the pandemic.

“I don’t want to encourage people getting sick,” said Stanley S. Hubbard, a Minnesota billionaire who has donated more than $2 million to help Republicans, including President Trump, since the beginning of the 2016 election.