BOISE - A state lawmaker contends that Idaho's tax collectors risk violating the U.S. Constitution by requiring same-sex couples who are legally married elsewhere to do extra work when filing their state income taxes.

Boise Democratic Rep. John Gannon, a lawyer, says litigation in Ohio suggests Idaho's new rules requiring married gay couples to recalculate state taxes as singles after filing joint federal returns could be vulnerable, based on equal protection guarantees in the 14th Amendment.

In Ohio, a federal judge ordered the state to recognize a gay couple's legal marriage in New York - despite Ohio's constitutional ban. His rationale: Ohio has historically recognized other kinds of marriages between a man and woman completed elsewhere but otherwise illegal in Ohio.

Idaho also generally recognizes opposite-sex marriages legally contracted beyond its borders, unless they violate state public policy. That includes common law unions and marriages between first cousins that are tolerated in 19 states, but not Idaho.

"Idaho accepts other states' marriage definitions," Gannon said. "That creates an equal protection problem" when Idaho chooses to treat same-sex couples who are legally elsewhere differently during tax season, he said.

Last week, Idaho's Tax Commission changed its 2013 income tax instructions following June's U.S. Supreme Court decision invalidating parts of the federal Defense of Marriage Act. Now, its requirement for a taxpayer's Idaho filing status to match his or her federal filing status "does not apply to same sex couples who file a joint federal return; the State of Idaho does not recognize same sex marriages," according to the new advisory.

In 2006, Idaho voters backed an amendment to the state constitution designating marriage between a man and a woman as "the only domestic legal union." Additionally, a separate law states same-sex marriages are among those that violate Idaho's public policy.

After consulting with the Tax Commission's in-house lawyer, agency spokeswoman Liz Rodosovich said Tuesday that her agency doesn't share Gannon's concerns and believes its new rules are constitutionally sound.

Bob Cooper, a spokesman for the Idaho attorney general's office, said his office hasn't been asked by state lawmakers for a legal opinion.

However, in Ohio, a state with a similar gay marriage ban, U.S. District Judge Timothy Black ruled in July that the death certificate of a man suffering from Lou Gehrig's Disease state he was married and list his male partner as his surviving spouse because they were legally married in New York.

Black wrote that Ohio law has long recognized out-of-state marriages as valid as long as they were legal where they took place.

"How then can Ohio, especially given the historical status of Ohio law, single out same-sex marriages as ones it will not recognize?" he wrote. "The short answer is that Ohio cannot."

Gannon contends that Black's rationale could be shared by Idaho federal judges if the Tax Commission's rules are challenged. "The legal bills are going to be very expensive," he predicted.

Ohio isn't the only place where such issues are being litigated. In Pennsylvania, two women legally married in Massachusetts have challenged their state's gay-marriage ban, arguing they're being denied benefits - including the ability to file joint state taxes.

Monica Hopkins, executive director of the American Civil Liberties Union in Idaho, said Monday that she's begun inquiries within her organization about the Idaho Tax Commission's new rules.

Though she said it's premature to conclude whether they'd withstand a challenge, Hopkins estimates dozens or even hundreds of same-sex couples who are legally married in 14 states that allow gay unions but who live in Idaho will likely face recalculating state taxes in 2013.