“A lot of people, not just Democrats, are going to start looking to Mr. Biden and sizing him up. It’s a real-time test at some level,” said Addisu Demissie, who managed the presidential campaign of Senator Cory Booker of New Jersey. “Even though he doesn’t have authority, he can show and not tell what four years of a Biden presidency would feel like.”

In particular, Democrats believe the virus will help them hold more moderate, independent suburban voters who don’t like the president’s tone but have stayed with him because of the strong economy. The party won control of the House in 2018 largely on the strength of their support among those voters, flipping a number of seats in battleground districts. Exit polling from Tuesday’s primaries showed that a majority of voters saw Mr. Biden as the candidate they trusted most to handle a major crisis.

For rural voters, who are more likely to vote for Republicans, the economic ramifications could affect their bottom lines, particularly farmers and oil workers who are already hurting from trade policies.

Yet, the fiercely partisan moment in Washington has scrambled the politics of unity that traditionally kick into place during times of national crisis. The partisan divide is so pervasive that it has affected not only people’s feelings about the president’s response, but also their fears about the virus itself.

Roughly six in 10 Republican voters nationwide said they were not particularly concerned that the coronavirus would disrupt their lives, according to a Quinnipiac University poll released this week. Democratic voters were twice as likely as Republicans to say they are concerned.

While privately concerned about the impact on the president’s re-election prospects, Republicans are largely following Mr. Trump’s lead in minimizing the worries over the coronavirus and blaming Democrats and the media for focusing on the deaths it has caused.

“One thing the press has not covered at all is the people who have really recovered,” said Senator Ron Johnson of Wisconsin, a Republican ally of Mr. Trump’s. “Right now all people are hearing about are the deaths. I’m sure the deaths are horrific, but the flip side of this is the vast majority of people who get coronavirus do survive.”