Craig, of Idaho, is asking the Minnesota Court of Appeals to let him withdraw his guilty plea to disorderly conduct stemming from a bathroom sex sting at the Minneapolis airport.
The ACLU filed a brief Tuesday supporting Craig. It cited a Minnesota Supreme Court ruling 38 years ago that found that people who have sex in closed stalls in public restrooms "have a reasonable expectation of privacy."
That means the state cannot prove Craig was inviting an undercover officer to have sex in public, the ACLU wrote.
The Republican senator was arrested June 11 by an undercover officer who said Craig tapped his feet and swiped his hand under a stall divider in a way that signaled he wanted sex. Craig has denied that, saying his actions were misconstrued.
The ACLU argued that even if Craig was inviting the officer to have sex, his actions wouldn't be illegal.
"The government cannot prove beyond a reasonable doubt that Senator Craig was inviting the undercover officer to engage in anything other than sexual intimacy that would not have called attention to itself in a closed stall in the public restroom," the ACLU wrote in its brief.
The ACLU also noted that Craig was originally charged with interference with privacy, which it said was an admission by the state that people in the bathroom stall expect privacy.
Craig at one point said he would resign but now says he will finish his term, which ends in January 2009.