Lawmakers who vote to attach a same-sex marriage ban to a tax reform bill would be violating their oath of office, according to Mark Kende, a constitutional law professor at Drake University.

Republican state Rep. Christopher Rants has filed an amendment to add language to a bill that would define marriage as between a man and a woman for tax code purposes. Such a provision would be unconstitutional, Kende said, because it essentially ignores the Supreme Court’s decision in Varnum v. Brien, which said the state’s laws cannot treat same-sex couples different from opposite-sex couples for the purposes of marriage. A constitutional amendment is the only legal way to overturn the court’s decision.

“Legislators take an oath to follow the law,” Kende said. “That applies even if one doesn’t like the ruling.”

The Iowa Constitution requires that anyone appointed or elected to public office take an oath “to support the constitution of the United States, and of this state.”  Both documents have been interpreted over the years to grant courts the right to review and, if necessary, strike laws that are considered “unconstitutional.”

This is the second time this month that Rants has tried to force debate on same-sex marriage. Last week, he failed at amending a Health and Humans Services budget to include an amendment to Iowa’s constitution banning same-sex marriage.

Debate is expected on the bill Tuesday or Wednesday.