There are definitive points in time in politics where an issue becomes, as James Carville and I called it in our 2009 book, “res judicata.” Put simply, serious people stop arguing about it. Think of global warming—more than two-thirds agree it’s happening—or evolution—only four in 10 think we didn’t evolve. At one point, these were raging debates. We’ll remember 2012 as the year that same-sex marriage became res judicata.

In just a few days, four states will vote on same-sex marriage ballot initiatives. At least one is likely to break anti-gay activists’ perfect 32-state streak. Meanwhile, courts across the country are striking same-sex marriage bans. Every court that’s heard a challenge to the Defense of Marriage Act so far has struck it down. Many of the judges who ruled against DOMA were appointed by unabashed court-packer President George W. Bush. If that doesn’t spell impending doom for DOMA, what does? Justice Ruth Bader Ginsburg has hinted that the Supreme Court will hear an appeal in one of these cases this term, and it’s hard to believe that the justices will rule differently than every other judge who’s considered DOMA.

For the first time in history, the president of the United States has taken an unambiguous public stance on gay rights and marriage. President Obama and the Democratic Party are campaigning on a platform that includes marriage equality. Meanwhile, Governor Romney and the Republican Party are doubling down on a culture-war strategy of which “traditional marriage” is their centerpiece. The fact that Obama and Romney are on opposite sides of same-sex marriage has raised the profile of gay marriage politics, but that’s not what makes this election a true showdown over gay marriage.

Tuesday will be Republicans’ last stand on same-sex marriage because they’re going for broke. In 2004, anti-gay marriage ballot initiatives in 11 states boosted far-right turnout for Bush. In 2008, Republicans successfully pitted gay-rights advocates against black and Latino voters over California’s Proposition 8 and won additional anti-gay victories that same year in Arizona, Arkansas, and Florida.

This year, Republicans are spending millions on national anti-gay marriage campaigns and state ballot initiatives all over again. It’s not Maryland they’re worried about. They’re after the White House. The GOP has embraced the principle of losing a battle to win the war. Even when conservatives are 20 points behind, state-level anti-gay campaigns keep marriage politically salient, draw in opinion leaders, and make national news. Their model? The Swiftboat Veterans for Truth, whose initial ad buy was just $500,000. Why? Romney desperately needs evangelicals to have a shot at the White House.

White, evangelical voters represent nearly 50 percent of the conservative base and made up nearly a quarter of the electorate in 2008. In his analysis of the Republican primary, Ryan Lizza wrote, “the single best predictor of whether Romney loses a state is the percentage of voters who describe themselves as evangelical or born-again.” This fall, 21 percent of these voters say they’re unlikely to vote as compared with 17 percent of the voting population overall. If Romney can’t reclaim these voters, his Election Day performance and that of the Republican Party will suffer from Virginia to Texas.

Far-right opinion leaders have been understandably suspicious of a would-be president who let gays marry under his watch in Massachusetts. One in four conservative evangelical voters have said they are “uncomfortable” with Romney’s religion. A number of conservative voters interviewed for one Reuters piece did not know that the Church of Jesus Christ of Latter-day Saints banned polygamy in 1890. Many were familiar with allegations of Romney’s father’s affiliation with a holdout Mormon community in Mexico that still practiced “plural marriage.”

Romney’s attempts at ingratiation have included McCarthy-like purges of gay staff and the reinstatement of controversial Republican rainmaker Ralph Reed, who publicly promised record evangelical turnout. The GOP went even further, launching a guns-blazing assault on same-sex marriage in an attempt to remedy Romney’s enthusiasm gap. Far from palling around with polygamists, the salvo proclaims, Romney is the last bulwark between America and gay marriage.

Team Romney will also actively attempt to use Obama’s declared support for same-sex marriage to get traditional Democratic voters, like voters of color and the elderly, to stay home. For example, the National Organization for Marriage is airing a radio ad in North Carolina featuring a prominent black pastor admonishing: “President Obama has turned his back on the values of our community with his strong endorsement of the homosexual movement.”

Republicans made Tuesday a de facto referendum on their party’s regressive, anti-gay culture-war strategy. And Republican voters are going to reject it—partly because Romney’s a fake who did support civil unions in Massachusetts, but mostly because that dog just doesn’t hunt anymore in U.S. national politics. A majority of Americans agree with President Obama and the Democrats, with 51 percent supporting legalized same-sex marriage and just 41 percent in opposition. Two-thirds of Americans think that the U.S. should accept homosexuality and get on with the business of mending our economy. Get ready to celebrate November 6, 2012, as Republicans’ last stand against gay marriage.