According to the conventional wisdom accepted by many in the mainstream media, women tend to vote Democratic more than men do because of the issue of abortion. Ramesh Ponnuru points out at National Review Online that the data do not support this belief. According to Gallup, about the same percentage of men and women describe themselves as "pro-life," and "59 percent of men said that abortion should be legal either in no circumstances or in only a few; 56 percent of women chose those responses."

Ponnuru also notes:

The gender gap doesn’t appear to shrink, or do anything consistently, for Republican politicians who are pro-choice, according to exit polls. Governor Scott Walker of Wisconsin, a strong pro-lifer, won 57 percent of men and 48 percent of women in 2010. Governor Brian Sandoval of Nevada, who’s pro-choice, won 57 percent of men and 49 percent of women. 

There's another good test case to see how a candidate's position on abortion might affect elections: California in 2010. That year, there were two center-right female CEOs running as the Republican candidates for governor and senator. Carly Fiorina, the senatorial candidate, was pro-life. Meg Whitman, the gubernatorial candidate, was pro-choice.

Fiorina got 42.2 percent of the vote, Whitman got 40.9 percent of the vote. So the pro-life GOP candidate ran 1.3 percentage points ahead of the pro-choice GOP candidate.

We have no idea why the pro-life candidate ran slightly ahead of the pro-choice candidate, but if opposition to abortion drives female voters away from pro-life candidates, one would expect that Whitman would have run ahead of Fiorina among women. In fact, according to the exit polls, 39 percent of women voted for Fiorina, and 39 percent of women voted for Whitman.

No one denies that there's a gender gap, but the abortion issue certainly doesn't seem to be causing it.