Republicans made almost universal gains in mail voting during the 2024 election, eroding a key Democratic advantage in nearly every state that tracks party registration, according to a data analysis by The New York Times.

The Republican rise in the use of mail voting was almost always accompanied by a drop in registered Democrats casting a mail ballot, allowing Republicans to make significant inroads in battleground states like Pennsylvania, red states like Florida and blue states like Connecticut.

The turnaround was remarkable given Republicans’ yearslong skepticism toward mail voting, fueled by President-elect Donald J. Trump’s false claims about fraud. The method was once widely embraced by Republicans in Southern and rural states but fell out of favor during and after the 2020 election as Mr. Trump and his allies argued that the only safe way to vote was in person on Election Day.

Making up ground in mail voting was a critical goal for Republicans heading into last year’s election, as G.O.P. strategists worried that the party had created an opening for Democrats. Election experts in both parties have long believed that it is best to bank votes early, shrinking the universe of voters to target on Election Day and limiting the effects of bad weather or other unforeseen circumstances on turnout.

Mr. Trump, apparently cognizant of the Republican warnings, softened his stance in the final months of the 2024 election but still declared that he wanted “single-day voting.”

Coupled with a Republican surge in early in-person voting, the G.O.P. improvement in mail voting cut heavily into what had been a major strategic advantage for Democrats. The development is yet another warning sign for the Democratic Party as it debates its failures and looks to the 2026 midterm elections. With dwindling structural advantages in addition to a fraying coalition, Democrats will need to improve their Election Day get-out-the-vote efforts, particularly among low-frequency voters.