But Democrats' hopes for using the crisis to expand voting by mail face firm Republican opposition, as well as significant logistical challenges. In some states, it would amount to a major revamp of their voting system just eight months before an election.

Vote-by-mail boosters already lost the first round of the fight. Democrats tried and failed to insert a broad mandate expanding voting by mail in the stimulus bill, a proposal that could cost as much as $2 billion. Instead, the bill included $400 million to help states adjust elections however they see fit before November.

But Democrats in Washington say they will keep pressing the issue, pointing to the increasing number of states that are shifting to mail-in voting for primaries as evidence that the time is right.

"Practically every single Tuesday, we see another state reacting to their inability to run their election in the middle of this incredible health care pandemic," said Democratic Sen. Ron Wyden of Oregon, the first state to vote entirely through the mail. He called expanded mail voting "not even a close call."

Former Vice President Joe Biden, the leading Democratic presidential candidate, joined the push Sunday. "We should be looking to all-mail ballots across the board," Biden said Sunday on NBC's "Meet the Press." "We should be beginning to plan that in each of our states."