The law firm King & Spalding reversed course Monday and announced it would not defend the Defense of Marriage Act on behalf of the House, marking a major victory for gay rights activists.

King & Spalding Chairman Robert Hays Jr. wrote Speaker John Boehner (R-Ohio) a letter to notify him of the decision. Hays said he “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 wrote.

Minority Leader Nancy Pelosi (D-Calif.) cheered the decision. In a statement, spokesman Drew Hammill said, “Leader Pelosi shares Mr. Hays’ apparent concerns with the lack of transparency and accountability in the way this contract was signed.”

The Atlanta-based firm and former Solicitor General Paul Clement, the lead attorney hired to defend DOMA, were the subject of fierce criticism from Democrats and gay rights activists over the past week.

Pelosi sent a series of probing letters to Boehner in which she asked the Ohio Republican how much the legal action would cost, who would pay for it and what the process was for selecting King & Spalding to defend DOMA in court.

Boehner hired the law firm last week to defend DOMA, the 1996 law that defines marriage as between one man and one woman, in a series of pending court cases. Boehner’s decision, on behalf of the Bipartisan Legal Advisory Group, came in response to President Barack Obama’s announcement last month that his administration would no longer defend the law in court.

Clement, whom Pelosi aides blasted in a series of releases last week, also resigned from his post at King & Spalding on Monday and announced he is joining Bancroft as a partner. The Washington-based firm was founded by Viet Dinh, who served as assistant attorney general in the Bush administration from 2001 to 2003. Bancroft is expected to take on the DOMA cases; Clement will serve as lead attorney.

In a letter to Hays, Clement also questioned the firm’s vetting process for taking on clients.

“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,” Clement wrote. “Defending unpopular positions is what lawyers do.”

Added Clement: “Much has been said about being on the wrong side of history. But being on the right or wrong side of history on the merits is a question for the clients. When it comes to the lawyers, the surest way to be on the wrong side of history is to abandon a client in the face of hostile criticism.”

Boehner spokesman Brendan Buck said, “The Speaker is disappointed in the firm’s decision and its careless disregard for its responsibilities to the House in this constitutional matter.”

Buck did offer praise for Clement, who will continue to represent the House in pending DOMA cases.

“Mr. Clement has demonstrated legal integrity, and we are grateful for his decision to continue representing the House,” Buck said in a statement. “This move will ensure the constitutionality of this law is appropriately determined by the courts, rather than by the President unilaterally.”