The United States Capitol dome at dawn. (Photo by Jonathan P. Larsen / Diadem Images)

The leader of a group dedicated to promoting the separation of church and state filed a lawsuit Thursday against House Chaplain Rev. Patrick Conroy after he rejected a request to deliver a non-religious invocation on the House floor.

Dan Barker, president of the Madison, Wis.-based Freedom From Religion Foundation, alleges that Conroy denied him an opportunity to deliver a guest invocation because Barker is an atheist. The lawsuit was filed in D.C. District Court on Thursday, the same day designated as the National Day of Prayer.

Barker also named Speaker Paul D. Ryan (R-Wis.) and several members of Conroy’s staff in his suit in an effort to persuade the court to force the chaplain to allow him to address the House.

“I would really love the opportunity to participate in solemnizing Congress,” Barker said in an interview. “We hope that I, or an atheist, be allowed to deliver a guest invocation before Congress.”

The Office of the House Chaplain did not respond to several requests for comment.

The lawsuit is the result of a year-long effort led by Barker and the Freedom From Religion Foundation and reopens a long-standing fight over whether it is appropriate for a religious leader to open the daily session in Congress.

Barker may face a difficult time getting a court to rule in his favor, said Robert A. Destro, director of the law and religion program at the Columbus School of Law at Catholic University, because Congress has wide latitude under the Constitution to determine how it operates.

“This is a fundamental separation of powers problem,” Destro said. “The House and Senate are authorized by Article I to make their own rules.  The rules governing what the chaplain does rest on that power.”

The House and Senate both employ full-time chaplains to offer pastoral services to members, oversee the Congressional Prayer Room and occasionally conduct religious ceremonies. Conroy was nominated House chaplain in 2011 by then-Speaker John A. Boehner (R-Ohio) and was elected by House members.

Conroy is responsible for delivering and overseeing an opening prayer that is delivered at the beginning of each daily session of the House. His office also coordinates a guest chaplain program where House members are invited to nominate clergy to fill the slot on an occasional basis.

Barker spoke with his congressman, Rep. Mark Pocan (D-Wis.), in early 2015 about sponsoring him for one of the available guest opportunities.

Barker claims that approximately 40 percent of the daily prayers on the House floor were delivered by guest chaplains between 2000 and 2015. None of the 857 guest chaplains during that time were atheists, according to the lawsuit.

Pocan wrote to Conroy on Feb. 18, 2015, notifying the chaplain of the request and outlining Barker’s plans to “offer the House of Representatives a hopeful invocation focusing on leading a happy, loving, moral and purpose-filled life.” Barker later submitted his formal biography and a draft of the invocation he intended to deliver.

The request was eventually denied according to a December 2015 email sent by Conroy’s office. Staff members said that all previous guest chaplains have been “practicing in the denomination in which they were ordained.”

Barker was ordained as a minister of Christ in the Christian Center Church, in Standard, Calif., in 1975, and served as a pastor at several churches before publicly announcing his atheism, in 1984.

“Daniel Barker was ordained in a denomination in which he no longer practices,” the email from Conroy’s office said.

Barker claims he was discriminated against for being an atheist. The lawsuit claims that Barker has retained his ordination and has used the credentials to officiate weddings, including a ceremony at the U.S. Air Force Academy, in Colorado Springs.

Conroy explained to Pocan in a letter sent in January that it has been a long-standing requirement that guest chaplains must be “ordained by a recognized body in the faith in which he/she practices.”

“This is a substantive requirement — not a mechanical or check-the-box requirement,” Conroy wrote. “For example, I do not invite Member-recommended individuals who have obtained an Internet-generated ordination to serve as guest chaplains, even if they hold deep and long-standing religious beliefs.”

Pocan’s office declined to comment on ongoing litigation and is not expected to be named in the lawsuit.

Barker said he believes he fulfilled all of the requirements outlined by the chaplain’s office and feels the denial was “a slap in the face.”

“I am struck by the indignity of a government official deciding who is in and who is out on religious grounds,” he said.