Congressman John Conyers has been accused by another former aide of sexual harassment, the third accusation against him in the last week.
Deanna Maher, the Michigan Democrat’s former deputy chief of staff, said he made unwanted advances and groped her.
The 88-year-old former civil rights activist’s lawyer denied the allegations.
The House Ethics Committee has launched an inquiry into Mr Conyers, who is the longest-serving member of Congress.
Ms Maher, 77, told the Detroit News the alleged harassment began shortly after she joined Mr Conyers’ field office in suburban Detroit, which she ran between 1997 and 2005.
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She alleged the first incident happened in 1997 when she rejected his offer to stay in his hotel room in Washington and have sex.
A year later she said that while he was driving on a Michigan motorway, his hand was “trying to feel me up” by touching her leg and tickling her neck, which caused him to drive erratically.
“I was saved by the bell because we got pulled over by the police for the way he was driving,” she told the Detroit News.
In the last alleged incident, which Ms Maher said happened at a meeting in 1999, the congressman “put his hand up my dress and whispered in my ear, ‘I didn’t know you had such great legs'”.
She told the newspaper she did not quit because she needed the job.
“John Conyers is a powerful man in Washington, and nobody wanted to cross him”, she told the Detroit News.
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His lawyer, Arnold Reed, denied the latest allegation, telling the BBC in a telephone interview: “Congressman Conyers is looking forward to the process taking its course and has maintained that he is not guilty.
“He has not acknowledged any wrongdoing on his part.
“The accuser or accusers have the right to say what they feel has transpired from their perspective.
“Mr Conyers equally has a right to maintain that he has not harassed, sexually or otherwise, anyone.”
The BBC also contacted Mr Conyers’ congressional office, but did not receive an immediate response.
On Sunday Mr Conyers announced he was stepping down from the powerful House Judiciary Committee to focus on clearing his name.
Accusations surfaced last week when Buzzfeed News reported Mr Conyers had agreed a settlement for a former employee who accused him of harassment.
Mr Conyers acknowledged the 2015 payment of $27,000 (£20,000), but denied misconduct.
He is not the only Democratic lawmaker facing an ethics investigation after such an allegation.
Minnesota Senator Al Franken apologised on Monday after he was accused of unwanted kissing and touching.
Mr Franken said he did not recall the incidents as described by the women, but he did not contest their accounts.
In a Minneapolis Star Tribune editorial, the newspaper said his apology fails the “full candor test” and “he seems to be saying, ‘I’m sorry for what you think I did'”.
“Can one credibly apologise for acts without acknowledging they occurred?” the editorial board asked.
It added that the former comedian should “consider what is best for Minnesota and to weigh that more heavily than what might be best for his political career”.
