When news broke that King & Spalding was withdrawing from representing the House of Representatives in the Defense of Marriage Act (DOMA) lawsuits, and that former solicitor general Paul Clement was leaving the firm to continue that representation, I didn’t think much of it. Lawyers leave law firms all the time, typically for a constellation of reasons, constellations that usually include the North star of money and which sometimes include a star or two related to ethics or professional satisfaction. I’d imagine that money played some role in Clement’s decision to leave King and Spalding, too. It usually does when a star partner leaves a large law firm. 

The only element of the story that made it interesting was the total failure of King & Spalding’s vetting process. Large firms typically have a protocol for evaluating representation of major clients, particularly representation of controversial clients or controversial issues like DOMA and gay rights. King & Spalding should have recognized that their heart was not in defending DOMA, told Paul Clement as much, and called it a day, and they embarrassed themselves by accepting then rejecting the representation.
More than enough people wrote about that aspect, so I left it alone.
Now, however, another aspect of the debacle has come to the fore: the claim that King & Spalding breached its ethical duties to the House of Representatives by accepting representation and then withdrawing. The New York Times editorial board, which owns one of the tallest soap boxes in the world, claimed the move was unethical
King & Spalding had no ethical or moral obligation to take the case, but in having done so, it was obliged to stay with its clients, to resist political pressure from the left that it feared would hurt its business. Paul Clement, a former solicitor general who quit as partner in King & Spalding over the decision, said, “a representation should not be abandoned because the client’s legal position is extremely unpopular in certain quarters.”

Justice is best served when everyone whose case is being decided by a court is represented by able counsel. 

Others have piled on.
Let’s get one thing straight: once King and Spalding realized they could not serve as zealous advocates in defense of DOMA, the only ethical course was to withdraw from representation.
That’s not to say that lawyers can begin and withdraw representation at their leisure. The Rules of Professional Conduct word it differently, but lawyers are of course prohibited from leaving their clients in a lurch. You can’t jump out of the case one week before trial because you haven’t been paid. You can’t drop one client for their adversary because the adversary offers you a better deal (the so-called "hot potato" rule).
But that rule governing the timing and nature of withdrawal from representation plays second fiddle to the lawyer’s prime directive
An advocate, in the discharge of his duty, knows but one person in all the world, and that person is his client.
What, exactly, were gay rights supporters at the firm supposed to do when they read the fine print of the DOMA representation agreement

[P]artners and employees who do not perform services pursuant to this Agreement will not engage in lobbying or advocacy for or against any legislation … that would alter or amend in any way the Defense of Marriage Act and is pending before either the U.S. House of Representatives or the U.S. Senate or any committee of either body during the term of the Agreement.

Paul Clements’ new firm, which has a half-dozen lawyers, all presumably politically conservative, can abide by that. A firm the size of King & Spalding certainly cannot.
Lawyers have a duty to be zealous advocates; once the zeal is gone, the advocacy cannot continue.