Two same-sex couples in Texas have filed a federal lawsuit seeking to overturn that state’s eight-year-old constitutional amendment barring gay marriage, their attorney said.

Arguments could be heard as early as January in a federal court in San Antonio in the case trying to nullify the 2005 amendment, which says, “Marriage in this state shall consist only of the union of one man and one woman.”

San Antonio attorney Daniel McNeel Lane Jr., the lead counsel for both couples, said current Texas law banning same-sex marriage runs counter to the U.S. Constitution and is a “badge of inferiority.”

“Just as the judicial branch protected the fundamental right to marry and established that discriminatory laws could not prevent mixed-race couples from exercising that right, the courts again must step in to protect the marriage right,” Lane said Wednesday.

The suit was filed on behalf of Cleopatra De Leon and Nicole Dimetman, two women who live in Austin and were married in Massachusetts, and Victor Holmes and Mark Phariss, two men who were denied a marriage license when they applied last month in San Antonio.

A year ago, only six states — Massachusetts, Vermont, New Hampshire, New York, Connecticut and Iowa — plus the District recognized same-sex marriage, but the number has since more than doubled.

Texas Republicans, who dominate statewide offices and have the majority in the legislature, have stood behind the amendment as part of what they say is a defense of traditional marriage.

Texas Attorney General Greg Abbott, a Republican and a defendant in the case, is the front-runner in next year’s race for governor. He is a supporter of the law.

Gov. Rick Perry has said he “believes in the sanctity of marriage between a man and a woman, regarding it as the linchpin of the family unit and, thus, society as a whole.”