Voters cast their ballots during the U.S. presidential election in Medina, Ohio, U.S. November 8, 2016. REUTERS/Aaron Josefczyk

PHILADELPHIA (Reuters) - President Donald Trump told Republican lawmakers on Thursday he would work against illegal voting, continuing to insist it is a problem in the United States despite a consensus among officials that voter fraud is rare.

“We ... need to keep the ballot box safe from illegal voting,” Trump said at a party retreat in Philadelphia. “We are going to protect the integrity of the ballot box and we are going to defend the votes of the American citizen. So important.”

Trump said on Wednesday he would seek an investigation into illegal voting after being repeatedly challenged over his assertions that he lost the popular vote in the Nov. 8 election to Democrat Hillary Clinton due to voter fraud. He has not offered evidence to support the claim.