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“For Romney, the issue is reproductive rights,” Mr. Zogby said. “Does he really represent a party that is questioning contraception, talking ‘legitimate rape’ and rape-born pregnancies as God’s gift? It’s not surprising that there is an issue there for women.”

Whatever the underlying causes, the “gender gap” can be — and has been — electorally decisive. Four years ago, women voters supported Mr. Obama over Republican candidate Sen. John McCain 56 percent to 43 percent — essentially ensuring the Democratic candidate’s victory, given that Mr. Obama beat Mr. McCain among men by a single percentage point, 49 percent to 48 percent.

In Gallup’s final pre-election survey of likely 2012 voters, Mr. Romney held 49 percent of the vote to Mr. Obama’s 48 percent. Predictably, support for each candidate broke along sex lines, with Mr. Romney holding a 53 percent to 43 percent lead among likely male voters and Mr. Obama holding a 53 percent to 44 percent lead among women. In addition, a review of more than 70 October polls by Tufts University professor Richard Eichenberg found that Mr. Obama led among female voters in all 11 battleground states and was running ahead of his margin of victory among women in Florida, Iowa, Ohio, Michigan and Virginia.

Given his declining support among male voters, Mr. Obama’s Election Day strategy was clear: turn out women and protect his advantage against Mr. Romney’s best efforts to shrink it.

“There’s a reason you see the Obama campaign micro-targeting female voters,” Ms. Lawless said. “The Democratic Party is very well aware that they need this gender gap to exist to win elections.”

All the single ladies

According to the nonprofit Voter Participation Center, the “gender gap” is actually shorthand for a bigger electoral asymmetry: a “marriage gap” among women themselves.

In 2004, unmarried women voted for Democratic candidate Sen. John F. Kerry over incumbent President George W. Bush by a 25 percentage point margin. Four years later, Mr. Obama enjoyed a whopping 44-point margin over Mr. McCain among unmarried women — essentially accounting for the entire “gender gap,” given that Mr. McCain won married women by 4 points.

“Unmarried women were 23 percent of the electorate in 2008 and are the fastest-growing big demographic group,” said Page Gardner, president of the Voter Participation Center. “The way that they vote and the margins they give candidates have been and can be absolutely determinative. You see that not only in the top of the ticket, but also in senatorial and gubernatorial races.”

On the eve of the election, Mr. Zogby was more blunt.

“The whole election boils down to one factor,” he said. “Single women. Obama certainly leads with them, but the real issue is that about 10 percent of them were undecided. So the questions are, why and will they vote?”

Both presidential candidates spent much of the campaign attempting to address that question, albeit in different ways. Mr. Obama touted his signing of the Lilly Ledbetter Fair Pay Act — a federal law that makes it easier for women to file pay-discrimination lawsuits — as well as a provision in the federal health care law that requires insurers to provide free birth control.

Mr. Obama also attacked Mr. Romney for being noncommittal on whether he would have signed the Ledbetter law as president, pledging to end federal funding for Planned Parenthood and for backing Indiana Republican Senate candidate Richard Mourdock, who drew widespread criticism for stating that if a pregnancy results from rape, that life is still “something God intended.”

Allison Kellogg, a 50-year-old loan servicer from Henrico County, Va., said those issues played a crucial role in her decision to vote for Mr. Obama.

“That was the main reason, Romney’s position on women’s rights as far as abortion and birth control,” she said. “Game over. You can have your own opinions on those, but they’re not topics for legislation. And I would feel the same way if Barack Obama started spewing the same foolishness that I feel the Republicans do.”

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