Gov. Jay Nixon (D), speaking Aug. 15, 2013, at the Missouri State Fair in Sedalia. (Orlin Wagner/Associated Press)

Missouri Gov. Jay Nixon (D) said Thursday that he would sign an executive order to allow gay and lesbian couples who were legally married in other states to file joint tax returns with the state Department of Revenue, a move likely to prompt a legislative reaction from the Republican-dominated legislature.

Nixon told reporters Thursday that because the couples will be able to file joint returns with the Internal Revenue Service, the Missouri Department of Revenue should accept those returns as well.

The Treasury Department and the IRS ruled in August that legally married same-sex couples could file joint returns after a Supreme Court decision in June overturned a provision of the Defense of Marriage Act.

Nixon, a conservative Democrat serving his second term, said the move wasn’t about defining marriage. He said because taxpayers have to file federal tax forms with the state Revenue Department, allowing gay couples to file with the state “is the only appropriate course of action.”

“Many Missourians, including myself, are thinking about these issues of equality in new ways and reflecting on what constitutes discrimination. To me, that process has led to the belief that we shouldn’t treat folks differently just because of who they are,” Nixon said Thursday, according to the St. Louis Post-Dispatch. “I think if folks want to get married, they should be able to get married.”

Missouri’s Constitution defines marriage as between one man and one woman, and Republican officials accused Nixon of acting unilaterally to appease the Democratic base.

“The governor’s job is to defend our state’s constitution — including the constitutional amendment defining marriage as between one man and one woman that was passed overwhelmingly in this state — not to surrender to the whims of the Obama administration,” state House Speaker Tim Jones (R) said in a statement first reported by the St. Louis Post-Dispatch.

The executive order doesn’t allow gay and lesbian couples with a marriage certificate to take advantage of state tax breaks already available to straight couples.