On one aspect of whether same-sex couples should have the right to marry, both sides agree: The issue defines what kind of nation we are. Half a dozen states and the District of Columbia have made history by legalizing it, but it's prohibited elsewhere, and 30 states have placed bans in their constitutions.
Where they stand:
President Barack Obama supports legal recognition of same-sex marriage, as a matter decided by states. He's also repudiated the Defense of Marriage Act, which denies federal recognition of same-sex marriages and affirms the right of states to refuse to recognize such marriages. The administration no longer defends the law in court, but it remains on the books. Republican Mitt Romney says same-sex marriage should be banned with a constitutional amendment, not left to states. He also opposes civil unions if they are equivalent to marriage, and says states should decide what rights and benefits should be allowed for same-sex couples.
Why it matters:
The debate divides the public down the middle, according to recent polls, and stirs up passionate feelings on both sides.
Those who oppose it often invoke religious teachings, contending that their faith cannot condone legal recognition of gay and lesbian couples. They worry about conflicts between religious liberty and public policy if gay marriage spreads to more states and gains federal recognition.
Gay-marriage supporters cite examples of devoted same-sex couples — some partners for decades, some raising children — and say it's unfair to deny them the same rights as heterosexual couples.
"It's about what kind of country we are," said Lee Swislow of Gay & Lesbian Advocates & Defenders, a Boston-based legal group that has won several landmark court rulings. "Do we treat each other the way we want to be treated?"
"The average citizen of Maryland has enough common sense to know that marriage cannot be redefined; that a child comes from both a mother and a father; that marriage is the building block of society," the group said. "It is not discriminatory to reserve marriage for one man and one woman."
As things stand now, same-sex couples face a patchwork of conflicting laws and practices that vary from state to state.
Six states allow same-sex marriage; nine more have civil unions or domestic partnerships that extend marriage-like rights to gays and lesbians.
The federal government, however, doesn't recognize same-sex marriage, nor do the vast majority of states. Even with an out-of-state marriage license, gay and lesbian couples in those states face uncertainty, extra legal bills and rebuffs that straight couples avoid. Complications can arise with adoptions, inheritances and survivor benefits.
If legally married in their own state, same-sex couples still must file separate federal tax forms, with separate deductions, even when they're raising children together and jointly owning property.
This election won't get rid of that patchwork, but it could have a major impact given that four states have gay-marriage measures on their ballots.
Thus far, foes of gay marriage have prevailed in all 32 states where the issue reached the ballot. If that streak is broken in the four states that are addressing it in November, it could provide momentum for supporters, and perhaps even influence the Supreme Court if — as expected — it takes up cases challenging the Defense of Marriage Act.