A prominent law firm hired by Republican lawmakers to defend the federal ban on gay marriage said Monday it was withdrawing from the case amid criticism by advocacy groups, prompting the partner leading the work to quit.

The move by Atlanta-based King & Spalding is the latest flashpoint in the public debate over gay rights. Chairman Robert Hays Jr. said the firm chose to divorce itself from the controversy after determining that the decision to take the case wasn't vetted properly, but gay rights groups had also been pressuring the 800-lawyer company with plans for a protest Tuesday in Atlanta and with calls to its other clients. The groups cheered the move.

The decision, however, was sharply criticized by conservative groups, legal observers and the partner who had been handling the case, a former high-ranking Justice Department official under President George W. Bush. Washington-based attorney Paul Clement said he's moving to another law office so he can continue the work.

Clement had been retained by House Republican leaders after President Barack Obama ordered the Justice Department in February to stop defending the Defense of Marriage Act. His administration said it believes the 1996 law, which defines marriage as only between a man and a woman, was unconstitutional.