Their frontrunner candidate has dementia and can't debate Trump, and 2nd place can't beat Trump in the general election.
They'll try to say that an election during this crisis is unfair, and that it must be postponed - and then Trump is over a barrel. If he postpones it, they'll call him a dictator, and if he doesn't they'll say it was rigged against them.
Best case scenario, one of the proposed treatments proves effective and this blows over in the next 2 weeks.