Marc Elias, a Washington lawyer who leads the Democratic National Committee’s legal efforts to fight voter suppression efforts, said not a day goes by when he doesn’t field a question from senior Democratic officials about whether Mr. Trump could postpone or cancel the election. Prodded by allies to explain why not, Mr. Elias wrote a column on the subject in late March for his website — and it drew more traffic than anything he’d ever published.

But changing the date of the election is not what worries Mr. Elias. The bigger threat in his mind, he said, is the possibility that the Trump administration could act in October to make it harder for people to vote in urban centers in battleground states — possibilities, he said, that include declaring a state of emergency, deploying the National Guard or forbidding gatherings of more than 10 people…

Ms. Brooks’s group at Georgetown is hardly the only one forecasting doomsday scenarios for the election. Ian Bassin, the executive director of Protect Democracy, a nonprofit group dedicated to resisting authoritarian government, last year convened the National Task Force on Election Crises, a bipartisan 51-member group that includes Republicans such as Michael Chertoff, the former homeland security secretary. The group is dedicated to envisioning and presenting plans for scenarios that could wreck the 2020 presidential election.

The task force began with 65 possibilities before narrowing the list early this year to eight potential calamities, including natural disasters, a successful foreign hack of voting machines, a major candidate’s challenging election results and seeking to delegitimize the results, and an incumbent president who refuses to participate in a peaceful transfer of power.