Washington, D.C. ? The American Civil Liberties Union has again come to the defense of U.S. Sen. Larry Craig, with a friend-of-the-court brief that argues police violated the constitutional rights of the Idaho Republican when he was arrested in a sex sting last summer.

In its brief, filed Tuesday with the Minnesota Court of Appeals, the ACLU argues that there’s nothing illegal about soliciting private sex in a private place – in Craig’s case, from an undercover police officer in a bathroom stall at the Minneapolis-St. Paul Airport.

Craig’s actions were constitutionally protected free speech, the ACLU argues. “Because the government may not criminalize private sex, it may not criminalize an invitation to have private sex,” the ACLU brief says. “This is even so where the invitation is extended in a public place, whether a bar or a restroom.”

Craig has filed a brief of his own seeking to overturn a lower court decision that said he couldn’t withdraw his guilty plea to disorderly conduct. Craig pleaded guilty in August to disorderly conduct after he was arrested in the men’s room sex sting in June.

In the brief, Craig’s lawyers argued that the airport police failed to prove that the senator engaged in disorderly conduct.