Worse for Democrats, the coronavirus pandemic disrupted the usual pathways that the party had used to bring in new voters: sign-ups at the Department of Motor Vehicles and face-to-face field work. And it comes as President Biden faces growing skepticism among African Americans over whether he has a formula to overcome voting restrictions pushed by G.O.P.-led state legislatures — the topic of a high-profile address that he plans to give on Tuesday in Atlanta.

An analysis by Catalist, a Democratic data firm, shows that in 2020, the Democrats’ traditional edge in voter registration shrank to nine percentage points in key states, down from a 19-percentage-point advantage over Republicans in 2009.

The overall picture might not be so grim for Democrats, because newer registrants are still leaning Democratic at typical rates, even if they register as independents. The trend has nonetheless alarmed some party insiders.

“There’s a big deficit building up on the Democratic side, which could start becoming consequential,” said Michael Podhorzer, a strategist and former co-chair of Catalist.