Paul Clement, former U.S. Solicitor General (Photo via Wikimedia Commons).

According to the firm's chairman, Robert D. Hays, Jr., that the firm did not consider all sides when it took on the defense.

From The New York Times:

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

The firm’s abrupt reversal highlights the continuing potency of same-sex marriage as a complicated issue that has scrambled traditional political calculations in Washington. President Obama has often called the marriage act “abhorrent,” but his Justice Department defended it for more than a year before declaring it unconstitutional. Attorney General Eric H. Holder Jr. announced in February that the administration would no longer defend the act in court.

The move prompted the resignation of one of the firm's prominent partners, Paul Clement, a former solicitor general under the Bush administration. Clement left King & Spalding saying that a law firm should not give into criticism and pressure.

The Wall Street Journal reported:

"I resign out of the firmly held belief that a representation should not be abandoned because the client's legal position is unpopular in certain quarters," Mr. Clement said in a letter to King & Spalding's chairman, Robert D. Hays. "Defending unpopular decisions is what lawyers do."

Mr. Clement said he would be joining Bancroft PLLC, a small firm in Washington, D.C., where he will continue working on the case.

Same-sex marriage is legal in five states and the District of Columbia. But under DOMA, the federal government doesn't recognize such marriages, so people married under the state laws don't enjoy benefits granted to couples by the federal government.

In February, the Obama administration said new challenges to the law in New York and Connecticut forced it to take a deeper look at the law. Still, the administration ordered federal agencies to observe the law until Congress repeals it or a court decides its constitutionality.

The main opponent to King & Spalding's defense of DOMA was the Human Rights campaign, a national organization dedicated to fighting for equality, most recently the legalization of same-sex marriage. In light of King & Spalding taking on the case, the Human Rights Campaign announced a series of measures they would take against the law firm this week, all of which have been withdrawn.

From the Atlanta-Constitution Journal:

A protest near King & Spalding’s Atlanta headquarters, scheduled for Tuesday, has been cancelled. And a full-page ad denouncing the law firm in Tuesday's editions of The Atlanta Journal-Constitution has been pulled.

King & Spalding’s retreat was forced in part by its chosen emphasis on an open work force. Its own website says the firm is committed to employing the brightest and most diverse lawyers it can find, including members of the lesbian, gay, bisexual and transgender community.

The site notes that the Human Rights Campaign – which led last week’s attack -- gave the firm a 95 out of a 100 corporate equality index rating the past four years and that domestic partner benefits are offered for same-sex couples.