Joy Villa, a vocal Trump supporter who wore a dress bearing the president's "Make America Great Again" campaign slogan to the Grammy Awards this year, said on Tuesday she had contacted police in Washington on Sunday to file charges.
Villa told news website Politico and television channel CNN this week that Lewandowski touched her against her will while they posed for photographs at the Trump International Hotel.
Villa's representative has not responded to a request for comment.
Lewandowski could not be reached for comment, and Reuters has not independently verified the allegations.
Asked about the allegation in an interview on Wednesday, Lewandowski told the Fox Business television network that the matter was under investigation.
"There is a process which they will go through to determine a person's innocence," he said. "I'm not here to speak for Joy. What I am here to do is to speak for me. And what I am going to do is to let the process play forward."
Villa's account first appeared in an interview published by Politico on Friday.
"I'm wearing this silver suit and stretchy pants, and after the photo, he smacks my ass really hard," she told Politico. "It was completely demeaning and shocking."
She said she was hesitant to go to police at first, but on Sunday filed sexual assault charges.
Villa says she is considering running for a seat in the US Congress representing Florida.
Dustin Sternbeck, a spokesman for Washington's Metropolitan Police Department, said the allegations were still being investigated.
In response to questions about the case, the department provided reporters with a copy of the so-called "public incident report" which contained scant details and did not name either Villa or Lewandowski.
While still working as Trump's campaign manager, Lewandowski was arrested and charged with misdemeanour battery by police in Jupiter, Florida, after a reporter for Breitbart News said he grabbed and bruised her arm while she tried to question Trump at a campaign event.
Lewandowski, who has also worked as a lobbyist and a political consultant, denied he had grabbed or hurt the reporter, and prosecutors declined to proceed with the case. Trump dismissed Lewandowski from his position as campaign manager a few months later.
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