It happens that we’d been thinking about the courage of a certain kind of lawyer who sticks with an unpopular client even when attempts are made to drive him off a case. For we’d just gone to see Robert Redford’s powerful movie called “The Conspirator,” about Mary Surrat, who went to the gallows for her role in the plot to assassinate President Lincoln. It centers on her lawyer, Frederick Aikin. He was reluctant to take her case, but took it on principle. He was reviled for doing so, but he stuck with the case and came to have doubts about her guilt. When she was hung, he left the bar altogether and became the founding managing editor of the Washington Post.

That is the kind of grit being shown by Paul Clement, the former solicitor general of the United States who just quit the prestigious law firm of King & Spalding to stick with his promise to represent the House of Representatives in its fight with the government over the Defense of Marriage Act. The House is seeking to challenge the decision of the Obama administration not to enforce the law, which defines marriage as a union between only a man and a woman and prohibits states from being forced to recognize same gender unions made in other states.

The Obama administration’s announcement in February that it would no longer defend the act in court was startling, given that the attorney general of the United States tends to defend even those laws an administration doesn’t like. We don’t gainsay the responsibility of Mr. Obama to make his own judgment in respect of whether a law he’s being asked to sign or to enforce is constitutional. We’re not so sure that in the end the courts themselves will differ with President Obama’s conclusion about the constitutionality of the law. But the House is sticking by the law it passed, and it engaged Mr. Clement, who had been solicitor general in the latter years of the George W. Bush administration.

No sooner had Mr. Clement stepped up to represent the House of Representatives, however, than King & Spalding come under what the Huffington Post called “intense criticism” from lesbian, gay, bisexual, and transgender rights advocates, and the law firm petitioned to be allowed to withdraw from the case. That was when Mr. Clement announced he would resign from the firm to stick with his commitment to represent the House. “My thoughts about the merits of DOMA are as irrelevant as my views about the dozens of federal statutes that I defended as Solicitor General,” Mr. Clement wrote in his letter of resignation. “Instead, 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 positions is what lawyers do.”

The principle of sticking by an unpopular client is as old as our republic, whose second president, John Adams, when he was a young lawyer, won the acquittal of Captain Preston and most of his fellow redcoats in the killings that became known as the Boston Massacre. Most recently, some lawyers representing detainees at Guantanamo had come under criticism for being over-zealous in their defense. The lawyers were defended by President Bush’s last attorney general, Michael Mukasey, in an op-ed piece in the Wall Street Journal. The toll taken on lawyers who stick by unpopular clients is a remarkable thing, and Mr. Redford’s movie about Mary Surrat’s case gives a powerful insight. It would be fun to see who plays the part of Paul Clement should, a generation from now, Hollywood make an attempt to capture his courage