The news that Gov. Eliot Spitzer will soon introduce a bill to legalize same-sex marriage  what he calls “a simple moral imperative”  is welcome and could give new national momentum to this important cause. Mr. Spitzer would be the first governor in the nation to introduce a gay marriage bill. But if he is going to make a real difference, rather than simply checking off a box to fulfill a campaign promise, he will have to fight for the law vigorously.

Even in a progressive state like New York, this will be a steep political climb. So far, only Massachusetts has enacted a gay marriage law  after its highest court held that gay couples had a right under the State Constitution  and while there is a similar bill working its way through the Connecticut legislature, its prospects are uncertain. Civil unions or domestic partnerships involving same-sex couples are now recognized by a small but growing number of states, including Connecticut, New Jersey, Vermont, California, Hawaii and Maine. It is an indication of how big a challenge Mr. Spitzer faces that New York is not, and hasn’t come close to being, on this list.

Mr. Spitzer is right to be fighting for gay marriage. Civil unions and domestic partnerships are an important recognition of gay relationships by a state. But they still represent separate and unequal treatment. One federal study identified more than 1,100 rights or benefits that are accorded only to the legally married. That means that even in states recognizing civil unions and domestic partnerships, gay couples often have to use legal contortions to protect their families in ways that married couples take for granted. Gay couples may also be discriminated against when it comes to taxes and pension benefits.

The next step in building momentum for gay marriage in New York will be to get the State Assembly, which has a Democratic majority, on board. Speaker Sheldon Silver has said he will not take a stand until he talks with his fellow Democrats. But most of those Democrats have already publicly expressed support for gay marriage, so Mr. Silver has no excuse to delay. He should make it clear that he will join Governor Spitzer and press for the legislation’s swift passage.