RIGHT CONCLUSION, WRONG RATIONALE?…. Late Thursday, Judge Joseph Tauro shook up the debate over gay rights, declaring a key provision of the Defense of Marriage Act unconstitutional. Supporters of equality on the left were thrilled; opponents on the right were not.

But it wasn’t long before observers took a closer look at the ruling itself. The left loved the outcome, but balked at the implications of the rationale. Many on the right loved the rationale, but resented it being used to further an equality agenda they find offensive.

A judge’s decision on Thursday declaring that a state law allowing same-sex marriage in Massachusetts should take precedence over a federal definition of marriage has exposed the fractures and fault lines among groups working to bolster states’ rights.

The decision, by Judge Joseph L. Tauro of United States District Court in Boston, supports and echoes a central tenet of the Tea Party, 9/12 and Tenth Amendment movements, all of which argue that the authority of the states should trump Washington in most matters not explicitly assigned by the Constitution to the federal government.

Congress, the judge said, had infringed on a question that was the province of local voters and legislators.

But in using the argument to support gay marriage in Massachusetts, where the case arose, the judge created an awkward new debating point within the less-government movement about where social goals and government policy intersect, or perhaps collide.

The article quotes a variety of conflicted far-right activists inclined to cheer a court ruling based on the 10th Amendment and promoting “states’ rights,” but equally inclined to denounce a court ruling that allows marriage equality.

Of course, the flip-side is true, too. Scott Lemieux explained the problem of going after DOMA on 10th Amendment grounds, rather than finding it unconstitutional on equal-protection grounds.

As Jack Balkin notes, if taken seriously Tauro’s holding that DOMA violates the 10th Amendment would have radical and extraordinarily reactionary consequences. I can understand the impulse to lay the hypocrisy of “state’s rights” Republicans bare. But even if you are a lot more sympathetic to the Supreme Court placing limits on congressional authority than I am, Tauro’s argument here is extremely weak. It’s far from clear why the federal government defining marriage for the purposes of its own laws represents an unconstitutional intrusion on the state of Massachusetts (which remains free under DOMA to recognize same-sex marriages for its own purposes).

The only way of making the case is if the federal powers implicated by definitions of marriage themselves are unconstitutional, an argument that would return us to an 18th-century night-watchman federal state. The cure in this part of Tauro’s opinion would be far, far worse than the disease.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.