The lawsuit asks that Nixon’s executive order be declared unconstitutional and asks for an injunction to stop Missouri officials from implementing the state tax code that way. A spokesman for Nixon had no comment Thursday about the court challenge.

Same-sex couples cannot marry in Missouri, and the state does not recognize such marriages conducted in other states where they are valid. Nixon in November directed officials to accept joint tax filings from same-sex couples who live in Missouri and have wed elsewhere. He said the order applied solely to tax filing status and does not authorize or sanction same-sex marriage in Missouri, where the constitution states marriages must be between a man and a woman to be valid and recognized.

The lawsuit filed Wednesday challenging Nixon’s directive asserted, “Governor Nixon has attempted to circumvent the clear public policy of this State mandating the historical legal definition of marriage, and to thwart the will of the People of the State of Missouri expressed in a recent Constitutional amendment as well as in State statues.”

Missouri’s tax code is tied to that of the federal government, and Nixon said married couples who file joint federal tax returns also must file state taxes jointly. The U.S. Treasury Department and the Internal Revenue Service last year determined legally married same-sex couples would be treated as married for tax filing purposes regardless of their residence.

Nixon said accepting joint state tax returns for couples who file their federal returns that way is appropriate, and the Missouri attorney general’s office has said the governor’s policy appears to follow requirements of Missouri tax law.

The lawsuit filed in the Capitol’s home of Cole County stated Nixon’s executive order forces the state Department of Revenue to require same-sex couples list one partner as “husband” and the other as “wife” on a return and therefore treat them as though they are legally married.

The suit was filed by Kerry Messer, who is the founder of the Missouri Family Network; Justin Mosher, who is a pastor and chairman of the Christian Life Commission of the Missouri Baptist Convention; Don Hinkle, who is director of public policy for the Missouri Baptist Convention executive board; and Joe Ortwerth, who is the executive director of the Missouri Family Policy Council.

Nixon said this fall he thinks that “if folks want to get married, they should be able to get married” and that he hopes voters have a chance to revisit whether to allow same-sex marriage. Missouri in 2004 became the first state to enact a constitutional amendment prohibiting same-sex marriage after the Massachusetts high court permitted gay marriage there. The Missouri measure passed with 70 percent of the vote.

A statewide organization that advocates for lesbian, gay, bisexual and transgender equality said Thursday it will work to ensure Nixon’s executive order is upheld.

“Gov. Nixon’s pragmatic and reasonable order ensures that Missouri income tax law continues to mirror the Federal Treasury and IRS,” PROMO executive director A.J. Bockelman said. “This order gives clear and equal guidance to all legally married couples in Missouri about how to complete their state income tax returns.”

The case has been assigned to Circuit Judge Jon Beetem. A hearing has not yet been scheduled.