President Trump plans to sign a directive on Thursday afternoon to move forward with an investigation into his claims of widespread voter fraud, according to the White House.

"Right now, it will be a follow-up on the announcement yesterday to better understand voter fraud,” press secretary Sean Spicer told reporters aboard Air Force One, as the president traveled to Philadelphia to meet with congressional Republicans at their retreat.

The spokesman did not provide further details.

The announcement follows Trump’s call for a probe into his baseless claims that between 3 million and 5 million ballots were illegally cast for Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE, preventing him from winning the popular vote.

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Trump’s focus on voter fraud has provoked concern in both parties.

Republicans in Congress have echoed academics, state attorneys general and even members of the president’s own legal team, who have said there is no evidence of widespread voter fraud in the 2016 election.

Democrats and civil rights groups worry the investigation could leak to a crackdown on voting rights.