GOP strategists around the country are concerned that President Trump’s vocal opposition to mail-in voting is resonating with Republicans, imperiling years-long efforts by the party to sign people up to vote by mail.

Expanded mail-in voting has become a pillar of modern political campaigns, enabling both Democrats and Republicans to secure millions of votes days or weeks before Election Day. Once voters are crossed off target lists, campaigns can narrow their focus to those who have not yet voted, conserving precious time and money in the closing days of an election.

“Whether we like it or not, increased vote-by-mail is going to be a reality this fall,” said Phil Cox, a former head of the Republican Governors Association. “Successful campaigns will need an integrated mail-in ballot strategy that blends messaging, modeling, and targeting. Anything short of that will be conceding votes.”

But Trump has wrongly claimed that mail-in voting opens elections to potential widespread fraud — even though he himself has voted by mail.

“Mail-In Voting, unless changed by the courts, will lead to the most CORRUPT ELECTION in our Nation’s History! #RIGGEDELECTION,” Trump tweeted Tuesday.

The president has never offered proof that mail-in voting leads to fraud. A commission Trump established in 2017 to hunt for voter fraud, led by Vice President Pence and former Kansas Secretary of State Kris Kobach (R), quietly folded after finding no proof.

But now it appears Republican voters are listening to Trump’s attacks. A Pew Research Center poll conducted last month found 83 percent of Democrats said voters should be allowed to vote early or by absentee without an excuse — compared to just 44 percent of Republican voters who said the same.

More than half of Republican respondents, 55 percent, said voters should only be allowed to vote early or by absentee if they had a documented reason for not participating in person on Election Day.

“It’s a huge issue,” said one Republican strategist who asked not to be named. “Republicans I talk to are worried. They’re seeing the effects of it already.”

As the primary season has progressed, Democratic voters have turned out in record numbers across the country, with many opting to vote by mail, hinting at a massive turnout in November. Republican turnout has been strong, though not earth-shattering.

No excuse is required to vote by mail in 34 states and the District of Columbia. The remaining states that do require an excuse include New Hampshire and Texas, two states likely to be competitive in this year’s presidential race.

Five states — Colorado, Hawaii, Oregon, Utah and Washington — conduct their elections entirely by mail. All but one county in California does the same, and several states conducted primaries this year entirely by mail to limit the risk of spreading the coronavirus.

A substantial body of research indicates that voting by mail does not give an edge to either party. In some states, Republicans tend to be overrepresented in vote-by-mail participation, while Democrats tend to favor in-person early voting. In others, that trend is reversed.

But if one set of voters cast ballots overwhelmingly by mail and another is discouraged from doing so, it may make the difference in November. Several Democratic groups, including one led by former first lady Michelle Obama, have launched campaigns to sign people up for absentee ballots.

Some Republicans have bucked Trump, either publicly or privately. Iowa Secretary of State Paul Pate (R) is slated to mail an absentee ballot request form to all active voters before November’s elections, after winning approval from state lawmakers last week. Other election administrators are considering doing the same after largely successful efforts during primary elections in states like Georgia and Nevada.

Texas Republicans used this month’s runoff elections to experiment with ways to persuade seniors to apply for mail-in ballots, though the data is still out, said one senior strategist behind the tests.

Trump’s own campaign has urged people to vote by mail, despite the president’s warnings. In an email sent to Pennsylvania Republicans, the Trump campaign pushed supporters to request an absentee ballot.

“President Trump is counting on you to vote for him and make sure he has HISTORIC voter turnout in Pennsylvania. Request your ballot and cast your vote from your own home,” the email reads.