Men and women differ in countless ways, from their interest in sports to their views on gun control to how rare they prefer their hamburgers. It’s no revelation that there are discrepancies in their voting habits too: Democratic candidates tend to get more support from women voters, Republicans do better among men. This is the vaunted “gender gap,” which has been a staple of presidential campaign coverage for at least three decades.

For some reason the notion took hold early on that this divergence would be a boon for liberals. “The gender gap is the Grand Canyon of American politics,” exulted Democratic strategist Ann Lewis in 1983. “It is wide, it is deep, and it is beautiful.” But that was wishful thinking: Of the last eight presidential elections, Democrats have won only three. The gender gap is real; it just doesn’t affect much.

Far more potent is the marriage gap in American politics. Married voters tilt Republican, while single voters favor Democrats. Back in 2004, USA Today advised readers of an easy way to predict whether a woman intended to vote for Kerry or George W. Bush: “Look at her ring finger.” Today that’s an even better rule of thumb.

A Quinnipiac University poll this month suggests just how important the marriage gap has become to President Obama’s reelection hopes. Among voters nationwide, it found, Obama’s advantage over Mitt Romney is narrow, 46 percent to 43 percent. Drill beneath the surface, however, and a sharp divide appears. Among married voters, Romney has a robust 13-point lead, 51 percent to 38 percent. But Obama enjoys an even larger lead among singles, 54 percent to 34 percent. Unmarried women in particular are in the president’s camp: They support him 2-1 over Romney.

It is that “yawning marriage gap,” Quinnipiac concludes, that is keeping Obama just ahead of Romney. The gap yawned even more cavernously in 2008, when Obama pulled an extraordinary 70 percent of the unmarried women’s vote. “I Got A Crush on Obama,” sang 26-year-old Amber Lee Ettinger in one of that year’s hottest internet sensations. The “Obama Girl” wasn’t the only one.

The political stakes are clear. Single women, one of the fastest-growing population cohorts, already account for one-fourth of all eligible voters. They are the “most reliably Democratic voting group outside African Americans,” writes Washington analyst Jessica Gavora, and coaxing them to the polls is an urgent priority for Obama’s strategists. That explains the Democrats’ whipped-up accusations of a GOP “war on women.” And it explains the Obama campaign’s “Life of Julia,” an internet slideshow that depicts a woman reaping the benefits of government programs at each stage of life. With help from “President Obama’s policies,” Julia is able to get an education, go to work, access health care, raise a child, launch a business, and retire in comfort. Everything, it seems, but get married.

What explains the marriage gap? Why, as Gallup editor-in-chief Frank Newport notes, is marriage a predictor of more conservative views, while being unmarried is a predictor of more liberal views?

One reason may be that people who are conservative to begin with are more likely to get married. This seems to be the case in religious circles. Higher marriage rates correlate with more frequent church attendance; religious commitment in turn is a strong predictor of party identification.

Yet if being conservative leads some people to marry, marriage may lead other people to become more conservative. Democratic pollster Anna Greenberg has observed that with marriage and parenthood often come experiences that tug voters rightward – they start to worry, for example, about kids’ exposure to sex and violence in popular culture.

“At the same time,” Greenberg wrote in 2006, “unmarried women are among the most marginal economic groups in the country… deeply concerned about their financial security.” With no spouse to provide financial protection, “Julia” may find the prospect of cradle-to-grave government support reassuring. The demographic that some Democratic strategists have dubbed SAFs – “single anxious females” – understandably tends to be more receptive to Democrats’ promises. Married women may be more inclined to likely to see the welfare state, with its soaring deficits and high taxes, as inimical to their families’ financial security. That drives more of them, in turn, to vote Republican.

These are only broad leanings, of course, not fixed rules. It’s easy to find liberal Democrats who are married and conservative Republican singles. Yet there’s little doubt that the marriage gap is a growing political force. The decline of marriage may not be healthy for society. But it’s certainly proving a windfall for the Democratic Party.

(Jeff Jacoby is a columnist for The Boston Globe. His website is www.JeffJacoby.com).