A judge temporarily blocked the agency from implementing a strict new rule governing mail voting. The policy could disproportionately affect Democrats and many battleground elections.

Voting by mail would become much more difficult, if not impossible, for tens of millions of Americans if the U.S. Postal Service is allowed to implement a restrictive rule governing mail voting that President Trump has demanded.

The effects would likely be greater for Democratic voters, who vote by mail in higher numbers than Republicans, and in states where mail voting is universal or nearly so. Mail voting may not happen at all in states that resist the Trump administration’s demands for voter data.

The rule would particularly affect people in rural areas and older voters, who tend to cast ballots by mail in greater numbers. It could also deeply affect states where some of the most competitive races for House, Senate and governor will determine control of Congress and state governments.

Michigan, where 37 percent of votes were cast by mail in 2022, would be one epicenter, with hard-fought statewide contests and multiple battleground House races. California, which mails ballots to all voters, is expected to be central to Democratic efforts to win a majority in the House.