King & Spalding announced Monday that it would no longer represent congressional Republicans in lawsuits over same-sex marriage, winning applause from gay-rights advocates while costing the firm its chief appellate lawyer, Paul Clement, who resigned in protest.

The fissure between the firm and Clement opened a week after gay and lesbian activists began criticizing the firm’s involvement in the suits. Clement signed a contract this month for the firm to represent House Republicans in arguing for the constitutionality of the Defense of Marriage Act, which defines marriage as between a man and a woman.

In a statement released Monday, King & Spalding chairman Robert Hays Jr. said the contract was a mistake. “In reviewing this assignment further, I determined that the process used for vetting this engagement was inadequate,” Hays said. “Ultimately I am responsible for any mistakes that occurred and apologize for the challenges this may have created.”

Clement, a former U.S. solicitor general under President George W. Bush, responded by scolding the firm for abandoning House Speaker John Boehner (R-Ohio) and others in the Republican leadership.

“I would have never undertaken this matter unless I believed I had the full backing of the firm,” Clement wrote in a two-page resignation letter to Hays. “If there were problems with the firm’s vetting process,” he added, “we should fix the vetting process, not drop the representation.”

In his letter, Clement quoted the late Griffin Bell, a former U.S. attorney general and King & Spalding partner, arguing that lawyers have a duty to finish a representation. “Sometimes you will make a bad bargain, but as professionals, you are still obligated to carry out the representation,” Bell said, according to the letter. (Bell, a Democrat, served as attorney general under President Jimmy Carter.)

Clement plans to continue representing lawmakers as they try to intervene to defend the marriage law. He is joining Bancroft, a small Washington litigation firm that fellow Bush administration veteran Viet Dinh founded in 2004. In another jab at King & Spalding, Clement said in a news release that Bancroft “offers its clients premier talent, without all the baggage of a mega firm.”

The development threatens to delay further several lawsuits that are related to the 1996 marriage law. In February, the Obama administration abandoned defense of the law and urged delays in a handful of cases to allow the House time to intervene. In court filings on Monday, the chairman of King & Spalding’s ethics committee, Richard Cirillo, wrote that the House may again need additional time as Clement moves to Bancroft.

Roberta Kaplan, a partner at Paul, Weiss, Rifkind, Wharton & Garrison, represents one of the plaintiffs, an 81-year-old widow in the Southern District of New York. Kaplan has said previously that she wants the case to move forward “as expeditiously as possible.” She declined to comment on Monday.

A spokesman for Boehner said the legal shakeup was mixed news. “The Speaker is disappointed in the firm’s decision and its careless disregard for its responsibilities to the House in this constitutional matter,” Brendan Buck wrote in an e-mailed statement. “At the same time, Mr. Clement has demonstrated legal integrity, and we are grateful for his decision to continue representing the House.”

King & Spalding’s decision came one day before the Human Rights Campaign, a national gay-rights organization based in Washington, planned to hold a news conference near the firm’s Atlanta headquarters. The news conference was to be part of a campaign to pressure the law firm among clients, recruits and the public. It’s now cancelled.

Joe Solmonese, president of the Human Rights Campaign, applauded the firm’s new stance. “King & Spalding has rightly chosen to put principle above politics in dropping its involvement in the defense of this discriminatory and patently unconstitutional law,” he said in a statement.

Dinh, Clement’s new law partner, said his firm would not react to similar pressure.

“If it is a turnoff to any of my clients, I would invite them to have a conversation with me about the essential role of lawyers, for whom it is utterly important to stick with their clients,” Dinh said. “Anyone who views this as a liability does not understand the responsibility of lawyers.”

Clement’s decision was totally unexpected and sent shock waves through the tight circle of Supreme Court advocates, several of whom declined to be quoted on record. It was like a surprise divorce being announced at a family holiday dinner, one said. It also leaves unanswered a myriad of questions about the status of Clement’s other cases and clients, and the future of King & Spalding’s D.C. appellate practice, which was built around Clement. For example, Clement is scheduled to argue on behalf of states challenging the Affordable Care Act before the U.S. Court of Appeals for the 11th Circuit on June 8.

He joined King & Spalding in 2008 amid reports that he was receiving a $5 million package for his services. Competitors also debated whether Clement’s high-profile move will attract clients seeking top talent with small-firm agility, or whether, at least temporarily, clients will shy away.

“Paul did nothing worse than act in the finest lawyers’ tradition” of taking on unpopular representations, said Arnold & Porter’s Lisa Blatt. She recalled that the profession was up in arms when lawyers were attacked for representing Guantanamo detainees. “Those were alleged terrorists, and this is an act of Congress,” said Blatt.

Dinh declined to discuss financial or other details of Clement’s hiring, though he said “it was a hard decision financially” for Clement. Dinh said he regarded his conversations with Clement as shielded by the attorney-client privilege, though he said Clement was not his client. “I regard all my conversations with friends who seek my advice as privileged,” he said.

David Ingram can be contacted at dingram@alm.com. Tony Mauro can be contacted at tmauro@alm.com.