Also on the ballot tomorrow: Initiatives that would legalize gay marriage in Maryland, Maine, and Washington State. These are a very big deal, and hint at the possibility of a long-term leap forward on civil rights.

According to a compilation of polls posted today by Slate, gay rights advocates are very likely on track to win in at least one of these states. Obviously they would prefer victories in all three. But a win in one, or better, in two, will still matter a great deal. It will mark the first time gay marriage has been ratified by popular vote, rather than via the courts or legislation.

That’s an important symbolic step, since it will confirm that the growing support for gay marriage we’re seeing in polls can be translated into success at the ballot box. But it goes beyond this. A popular vote victory on gay marriage could mean that the Supreme Court might be more open to arguments made by gay rights advocates on some of the major cases it may hear next year, ones that will go a long way towards determining the future of gay marriage in America.

Prominent gay rights advocate Richard Socarides notes to me that on Constitutional issues, the Supreme Court is generally hesitant to get out too far ahead of public opinion. Having gay marriage upheld by popular vote could help ease this hesitance. “Polls now show support for gay marriage, but we may have to prove this at the ballot box,” Socarides tells me.

The larger context here is that last month, a federal appeals court ruled the Defense of Marriage Act unconstitutional, and the Supreme Court is likely to hear arguments about it next year. Socarides notes that a positive showing at the ballot box tomorrow could make it somewhat more likely that the Supreme Court strikes down DOMA — which enshrines marriage as between a man and a woman for the purposes of federal law — for good. (California’s Pop 8 case may also come before the High Court next year, too.)

That’s not all. If one, two, or three of the initiatives passes tomorrow, that will add states to the six states where gay marriage is already legal. If gay advocates win two or three, you could be looking at a situation in which eight or nine states have legalized gay marriage — states that are home to a sizable chunk of the United States population. And if DOMA is struck down, that will mean full federal recognition for all of the gay married couples in all those states. That would be a very significant step forward for civil rights in this country.