The gender gap is a familiar feature of American politics and one of the reasons why President Obama remains competitive for reelection despite the weak economy. But there's another factor at work: it’s called “the marriage gap,” and it helps explain the tightening of the polls after the first presidential debate when a Gallup survey found that Romney had erased the gender gap, pulling within one point nationally of Obama among likely women voters.

Gallup’s findings prompted a swift response from Joel Benenson, the Obama campaign pollster, questioning the methodology, but other polls also found that Romney had substantially narrowed the gap with Obama among women. The leadership skills that Romney showed in the debate and his more-centrist positions weren't what people expected, says Republican pollster Ed Goeas. “For women especially, all of a sudden, the horns disappeared. He made the biggest gains with those who were negative towards him.”

The marriage gap was first noticed in 1992, when it became apparent that married women, especially those in the suburbs, tended to favor Republicans. Women who are unmarried for whatever reason—single, divorced, or widowed—see government in a more positive light and are more likely to vote Democratic. In 2008, Obama won women by 13 points and men by 1 point; he won unmarried women by 41 points, while John McCain won married women by 3 points.

That is the marriage gap at work, says Page Gardner, founder of the Voter Participation Center. This year’s tight race suggests that suburban women reassured by Romney in the debates have gone home to the GOP, but that Obama still has 63-64 percent support among unmarried women. “And that’s a very healthy marriage gap,” Gardner says. In focus groups conducted in Virginia and Ohio, responses from married and unmarried women differed markedly when they were asked about the secretly taped remarks where Romney appeared to disparage 47 percent of the country. “Honest” was how married women saw it, agreeing with Romney that “too many people are wanting the help of government.” Unmarried women thought Romney had it wrong, that the 47 percent are “trying to work their way and be better.” These women were offended by Romney’s words: “And so my job is not to worry about those people.”

Unmarried women are the main focus for the Obama campaign, but they’re not strong partisans. “They’re for Obama, it’s not a whim, but it’s not party identification,” says Democratic pollster Stan Greenberg. “If you’re looking for people who call themselves Democrats, it's not them.” Unmarried women were put off by inattention to their issues, particularly in the debates, says Greenberg, and that hurt Obama. “He’s getting back that base—young people and unmarried women—if they come back, he wins,” says Greenberg.

Reaching these women explains the Obama campaign’s strategy of highlighting Romney’s pledge to defund Planned Parenthood, and to talk about the issue of choice and abortion not as an issue of individual rights, but through the prism of economics. “Being able to control your economic destiny is also being able to control family size and access to contraceptives and health care,” says Gardner. Indiana Republican Richard Mourdock’s contention that rape victims should not have access to abortion, “is a wake-up call to a lot of women in this country, young and old.”

Unmarried women are a quarter of the electorate today, and of course, many are young, the leading edge of the millennial generation. The Obama campaign has been “very sophisticated throughout to reach the unmarried single young voter, and they really need to get their vote,” says Karlyn Bowman of the American Enterprise Institute, a conservative think tank. In a recent presentation about the changing American electorate, Bowman identified the quintessential female swing voter in the 1960s as a Dayton housewife who became so famous that according to political lore she even appeared on the popular Dick Cavett Show, cigarette in hand, right out of the "Mad Men" era.

In the groundbreaking book, The Real Majority, published in 1970, the Dayton housewife is described as 47 years old, Catholic, living in the middle of the country, middle-income, and married to a machinist. She had always voted Democratic, but by the ’60s was afraid to walk the streets alone at night, and she didn't like the Democrats’ cultural radicalism. Nixon speechwriter Patrick Buchanan brought the book to the president’s attention and a law-and-order campaign was shaped to capture the vote of that Dayton housewife.

Her counterpart today is more likely a single mother working at a low-wage job, reliant on government help to survive and maybe trying to get some education. Unlike their sisters in the suburbs, they are unreliable voters. Convincing these women that it is worth their while in a busy life to vote is a big part of the Obama ground game, and could make the difference on Tuesday in a tight race.