(CNN) -- Conservative Sen. Larry Craig got support from an unexpected source on Monday. The American Civil Liberties Union filed a brief in court saying the lawmaker's bathroom bust was likely unconstitutional.

The ACLU filed a brief in court saying Sen. Larry Craig's arrest was likely unconstitutional.

The ACLU urged a Minnesota District Court to let Craig withdraw his guilty plea.

"Sen. Craig has not always been a great friend of civil liberties, but you shouldn't have to endorse the civil liberties of others to keep your own," said ACLU Executive Director Anthony Romero, alluding to Craig's history of voting against gay rights.

Craig, R-Idaho, was arrested in June at Minneapolis-St. Paul International Airport in Minnesota after a police officer in a bathroom stall next to him alleged the senator attempted to solicit sex. Craig pleaded guilty August 8 to one charge of disorderly conduct. When the case came to light earlier this month, Craig announced his intention to resign September 30.

But days later, it was announced Craig would fight to overturn his conviction and may not resign. He filed papers September 10 to withdraw his guilty plea.

The ACLU friend-of-the-court brief was submitted to the Minnesota 4th District Court.

"The real motive behind secret sting operations like the one that resulted in Sen. Craig's arrest is not to stop people from inappropriate activity. It is to make as many arrests as possible -- arrests that sometimes unconstitutionally trap innocent people," Romero said in a written statement.

Don't Miss Craig files to withdraw guilty plea

Police must be able to demonstrate beyond a doubt that the sex was going to happen in public, he said. Regardless of whether it occurs in a bathroom or a bar, solicitation for private sex is protected speech under the First Amendment, the ACLU argues.

If the police really wanted to stop people from having sex in public bathrooms, they "should put up a sign banning sex in the restroom and send in a uniformed officer to patrol periodically," Romero said.

Patrick Hogan of the Metropolitan Airports Commission said Monday that authorities are prepared to defend the way Craig's arrest was carried out, as well as at least 41 similar arrests made this year in the same public restroom.

"Engaging in public sex in a bathroom is a crime and most people understand that without putting up a sign," Hogan said. "We saw a lot of communication about this particular bathroom on Web sites, and if we make it known that we're aware of it we can't be expected to enforce the law as effectively."

Hogan added, "We believe the charges fit the crime and Sen. Craig agreed to the charges as part of plea negotiations."

Craig has strongly denied he is gay. In an audiotaped police interview between him and the arresting officer, he repeatedly proclaimed he did nothing wrong.

The officer said Craig tapped his right foot on the floor, which the officer said he recognized "as a signal used by persons wishing to engage in lewd conduct," according to a police report.

The report alleges Craig then touched the officer's foot with his foot and the senator "proceeded to swipe his hand under the stall divider several times."

At that point, the officer said, he put his police identification down by the floor so Craig could see it and informed the senator he was under arrest, before any sexual contact took place.

"Government should make public restrooms safe for all, but it should do so in a manner that is really designed to stop inappropriate behavior, rather than destroying the lives of people who might have no intention of doing anything illegal," Romero said.



Craig has supported a federal constitutional amendment banning same-sex marriage. In 1996, he voted in favor of the Defense of Marriage Act, which denies federal recognition to same-sex marriages and prevents states from being forced to recognize the marriages of gay and lesbian couples legally performed in other states. Craig has also opposed expanding the federal hate crimes law to cover offenses motivated by anti-gay bias. E-mail to a friend

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