President Trump’s demonizing of voting by mail over the course of the past four years may have deprived him of a key tool in winning Pennsylvania this year, as Vice President Harris is opening a large lead with her mail-in effort.

Democrats in the state have so far returned their mail ballots at far higher rates than their Republican counterparts. 

Following the 2020 presidential contest that saw Trump lose his bid for re-election, in part, over mail-in voting in states like Pennsylvania, Wisconsin, and Michigan, the former president went to war with the very concept of mail voting — even though he himself voted by mail both in 2018 and 2020. 

“We want to go to paper ballots. We want to go to same-day voting. We want to go to citizenship papers. And we want to go to voter ID. It’s very simple. We want to get rid of mail-in voting,” Trump said at a rally in September. 

His urging of Republican voters to avoid mail voting altogether may be costing him critical support in Pennsylvania, where he is facing a huge deficit ballot requests compared to Ms. Harris. 

According to data posted by the Pennsylvania secretary of state’s office on Monday, nearly 70 percent of mail ballots that have been accepted by local and county voting officials have come from registered Democrats. Republicans make up just 23 percent of returned ballots, with the rest coming from voters who are not registered with a major party. 

Pennsylvania’s most Democratic counties are driving much of the mail-in ballot effort. In Alleghany County — home to Pittsburgh — registered Democrats have requested nearly 70 percent of all mail-in ballots, and nearly 75 percent of total ballots that have been returned so far. President Biden won Alleghany County with 59 percent of the vote in 2020, meaning Ms. Harris could be banking a large lead in a major metro area weeks before Election Day. 

Democrats are far outpacing Republicans when it comes to just requesting ballots, as well. More than 978,000 Democrats in Pennsylvania have requested mail ballots this year, with nearly 30 percent of them being returned so far. Republicans, on the other hand, have requested just more than 453,000 and have returned them at a rate of 21 percent. 

It seems Republicans have been caught lacking on the mail-in ballot front despite promising a robust effort to chase mail ballots earlier this year. The Republican National Committee set up a website called “Swamp the Vote” to encourage voters to request their mail ballots and to teach them how to ensure their votes are counted. Trump has continued to bash mail voting as fraudulent, though, even though he cut a short video for the Swamp the Vote website where he urges his supporters to mail in their ballots. 

“Any time you have a mail-in ballot, there’s going to be massive fraud,” Trump told television personality Phil McGraw in an August interview. The former president went further, saying the practice should be banned entirely. 

The RNC did not immediately respond to a request for comment about its mail efforts. 

None other than Elon Musk is now leaning into the mail ballot fight, even amid Trump’s criticisms. The Philadelphia Inquirer first reported on Friday that Mr. Musk’s America PAC was texting Pennsylvania voters who had not yet returned their ballots. 

“You got your PA ballot in the mail, but haven’t returned it yet. Trump needs your vote! Return it ASAP — the fate of our nation depends on you,” one text message sent to a Pennsylvanian read. 

Dean Browning, who runs a political group urging Republicans to vote by mail in Pennsylvania, told the Inquirer that Republicans just don’t have the infrastructure to get their voters to request ballots, trust them, and return them at the same rates Democrats do. Just two years ago, Democrats scored a 90 percent return rate on their mail-in ballots, while Republicans got a return rate of about 81 percent. 

“That’s a substantial difference,” Mr. Browning said. “Republicans used to say they don’t like mail-in. They just want to vote on Election Day. But the rules of the game have changed. If the Democrats have election month mailing in ballots, and we have just one day, we are not going to win in Pennsylvania.”