The gender gap was nearly absent in 1972 and 1976, the first two years the exit polls tested it. But after the Roe v. Wade decision in 1973, reproductive rights became a greater focus in presidential elections — particularly in 1980, when the Reagan campaign highlighted his opposition to abortion. The gender gap jumped to 17 points that year, with men much more likely to vote for Mr. Reagan.

The gender gap has often been widest when there is a Democratic president running for re-election, as in 1980 and 1996 (or a Democratic vice president looking to ascend to the presidency, as in 2000). Women, apart from their tendency to vote Democratic, also seem slightly more inclined than men to give the incumbent party another chance. When the incumbent is a Republican, as in 1976 and 1992, this can mitigate the gender gap.

If the current FiveThirtyEight forecast were recalibrated to show an overall nine-point lead for Mr. Obama — his lead among women in polls since the Denver debate — he would be a clear favorite in states totaling 347 electoral votes. Mr. Romney would be favored in states containing just 140 electoral votes. An additional 51 electoral votes would be too close to call.

About the opposite would happen if Mr. Romney led nationally by nine points — his current advantage among men. He would be all but certain to win states containing 321 electoral votes and would be highly competitive in traditionally blue-leaning states like New Jersey, Oregon and Washington.

The large gender gap exists despite the fact that men’s and women’s economic roles are becoming more equal — according to the Bureau of Labor Statistics, women represented 47 percent of the labor force as of September — and that women suffered at least as much as men in the recent economic downturn. The unemployment rate among women was 7.5 percent as of September; it was 7.0 percent when Mr. Obama took office in January 2009.

The unemployment rate among men is higher — 8.0 percent as of September — but it has declined rather than increased since Mr. Obama took office. It was 8.6 percent in January 2009 and peaked at 11.2 percent that year.

This suggests the gender gap instead has more to do with partisan ideology than pocketbook voting; apart from their views on abortion, women also take more liberal stances than men on social issues like same-sex marriage and gun control.