A Wisconsin group that advocates for separation of church and state is suing Congress for barring its co-president from giving an opening invocation before the U.S. House.

The lawsuit was filed Thursday, the National Day of Prayer. It alleges that House Chaplain Patrick Conroy rejected an application from Dan Barker, co-president of the Madison-based Freedom From Religion Foundation, to deliver a secular guest invocation.

Barker was sponsored by Democratic Rep. Mark Pocan of Wisconsin. The foundation says Conroy denied the request in January, saying Barker, who is an atheist, wasn't a true "minister of the gospel."

The complaint says Conroy is violating Barker's constitutional rights.

A spokesman for the House speaker's office didn't immediately reply to an email seeking comment.

The lawsuit was filed in U.S. District Court in Washington.