(Updated 2:50 p.m. Thursday, Jan. 9)

Four prominent conservatives, including former St. Charles County Executive Joe Ortwerth, have filed suit challenging Missouri Gov. Jay Nixon’s recent executive order to allow same-sex couples who have married in other states to file joint tax returns in Missouri.

Nixon said he did so because federal law also allows such joint filings, and Missouri law currently requires that the state tax filings mirror those filed with the federal government. Any couple who files a joint federal tax return is now required to file a joint state return. Nixon says that mandate would also apply to same-sex couples in Missouri who had been legally married elsewhere, and that his order simply clarified the situation.

The lawsuit asserts that Nixon’s order violates Missouri’s constitutional amendment, approved by voters in 2004, that bars same-sex marriages and defines marriage as between only one man and one woman.

Nixon has said that his order was not challenging Missouri’s gay-marriage ban, but the lawsuit asserts otherwise.

“The governor took an oath to uphold the Missouri Constitution, which includes our marriage definition,” said Michael Whitehead, the lawyer for the four filing the suit. “When he issued this order, he said he hoped the people of Missouri would take another look at recognizing same-sex ‘marriage.’ That is putting his thumb on the scales of justice.”

The case, which was filed Wednesday in Cole County Circuit Court in Jefferson City, asks that the courts toss out Nixon’s executive order and “declare that, in view of the state constitution, the Department of Revenue must not interpret tax laws to permit Missouri combined income tax returns from same-sex couples who were ‘married’ elsewhere.”

Whitehead said in an interview that the suit also challenges the validity of the state tax law that Nixon cited in his executive order.

Missouri is among 32 states that bar same-sex marriage; Whitehead said Missouri is the only one of those states to allow same-sex couples married elsewhere to file joint returns. Same-sex marriage is now legal in 18 states and in Washington, D.C.

A.J. Bockelman, executive director of PROMO, a Missouri advocacy group for gay rights, said in a statement Thursday, “Gov. Nixon's pragmatic and reasonable order ensures that Missouri income tax law continues to mirror the federal Treasury and IRS. This order gives clear and equal guidance to all legally married couples in Missouri about how to complete their state income tax returns. The plaintiffs have no grounds to file this case, the governor's order should stand."

Bockelman added in an interview that "all Nixon did was streamline the process'' for tax filings and that his order did nothing to change Missouri's gay-marriage ban. Those who filed the suit, he said, are "attacking the tax code."

A spokesman for Nixon pointed to the governor's original statement when he issued the order in November and indicated that the governor would have no further comment.

But Attorney General Chris Koster issued a statement Thursday afternoon reaffirming his agreement with Nixon's interpretation of Missouri's tax law.

"While we have not yet received a copy of the lawsuit, Gov. Nixon appears to be following the requirements of Missouri law on tax filing, as passed by the legislature," Koster's office said in a statement. "The attorney general's role is to defend such state laws to the extent possible and not to presume that our legislature's actions violate our state's constitution."

The plaintiffs are: