US Senator Larry Craig has denied being gay and has said he should not have pleaded guilty to disorderly conduct after being arrested in a men's toilet.
Sen Craig says he only pleaded guilty to speed up his case
The Republican senator was held in June by a policeman investigating complaints of lewd behaviour at airport toilets.
Speaking to reporters, Mr Craig said: "I am not gay. I never have been gay." He had done "nothing wrong", he added.
Republican leaders and a watchdog group have asked the Senate ethics committee to investigate Mr Craig's case further.
Mr Craig has already resigned from the 2008 presidential campaign of Mitt Romney, former Republican governor of Massachusetts.
Mr Romney said the senator had disappointed the American people, adding that his conduct reminded him of former President Bill Clinton's affair with White House intern Monica Lewinsky.
"Frankly it's disgusting," he told CNBC.
"He is no longer associated with my campaign ... I am sorry to
see that he has fallen short."
Mr Craig, a married father-of-three, said he would announce next month as planned whether he will run for re-election in 2008. He is in his third term as senator for the western state of Idaho.
In a statement issued on Monday, Mr Craig confirmed he had pleaded guilty to disorderly conduct three weeks ago.
But, the 62-year-old said, he should not have done so without taking legal advice.
"At the time of this incident, I complained to the police that they were misconstruing my actions," he said.
"In hindsight, I should not have pled guilty. I was trying to handle this matter myself quickly and expeditiously."
A charge of "gross misdemeanour interference to privacy" was dropped.
The watchdog group Citizens for Responsibility and Ethics in Washington filed a complaint with the Senate ethics committee on Tuesday.
According to a police report seen by the Associated Press news agency, Mr Craig entered a stall next to that occupied by an undercover policeman in the men's toilets at Minneapolis-St Paul airport.
According to the policeman, he tapped one foot several times and then moved it into the neighbouring cubicle to brush against the officer's foot.
The police officer recognised that "as a signal often used by persons communicating a desire to engage in sexual conduct", the official complaint said.
Mr Craig then gestured with his hand under the cubicle divider, AP quotes the document as saying, at which point the police officer identified himself.
Court papers show Mr Craig paid $575 in fines and fees and was put on unsupervised probation for a year, while a 10-day jail sentence was stayed, AP says.
Mr Craig has a conservative record in the Senate and has voted against gay rights and same-sex marriage legislation.
The furore comes only weeks after fellow Republican David Vitter admitted he had committed a "very serious sin" after his phone number was linked to an alleged Washington prostitution ring.