WASHINGTON – The law firm tapped by House Republicans to defend the Defense of Marriage Act abruptly backed out of the case on Monday, citing an "inadequate" vetting process for the engagement.

"Today the firm filed a motion to withdraw from its engagement to represent the Bipartisan Legal Advisory Group of the House of Representatives on the constitutional issues regarding Section III of the 1996 Defense of Marriage Act," said King and Spalding Chairman Robert D. Hays, Jr. in a statement published by Politico's Ben Smith and The Huffington Post's Amanda Terkel. "Last week we worked diligently through the process required for withdrawal."

"In reviewing this assignment further, I determined that the process used for vetting this engagement was inadequate. Ultimately I am responsible for any mistakes that occurred and apologize for the challenges this may have created," Hays said.

King and Spalding partner Paul Clement, the Bush administration's former solicitor general, was recently tapped by House Republicans to defend the controversial 1996 law that defines marriage as a union between a man and a woman. The firm faced pushback from LGBT activist groups who view DOMA as discriminatory and organized protests outside its Atlanta office.

The contract the firm signed with House Republicans, revealed last week, showed Clement and his colleagues would have been able to charge up to $520 an hour for the defense for a total of up to $500,000.

The Obama administration announced in February that it would no longer uphold the law.

UPDATE: Clement has resigned from King and Spalding, specifically citing the firm's decision to withdraw from the DOMA defense. In a letter to Hays first published by Politico, Clement wrote:

"I resign out of the firmly held belief that a representation should not be abandoned because the client's legal position is extremely unpopular in certain quarters. Defending unpopular clients is what lawyers do... I recognized from the outset that this statute implicate very sensitive issues that prompt strong views on both sides. But having undertaken the representation, I believe there is no honorable court for me but to complete it."