At the Honest Graft blog, Dave Hopkins writes:
The gender gap, produced by the relative pro-Democratic lean of women and pro-Republican lean of men in party affiliation and voting habits, has been a fact of American electoral life since the 1980s. In 2016, according to a study by the Pew Research Center, women voted Democratic for president by a margin of 15 points (54 percent to 39 percent), while men voted Republican by 11 points (52 percent to 41 percent). This difference was somewhat larger in 2016 than in other recent elections—probably reflecting the specific candidates on the ballot last time—though not dramatically so; Gallup estimated in 2012 that Barack Obama had carried the women's vote by 12 points while losing to Mitt Romney by 8 points among men.
But gender differences in the composition of the parties become greater as we move up the ladder of political engagement from average voters to activists, candidates, interest group leaders, and elected officials. Today, for example, 74 percent of female senators are Democrats, as are 73 percent of female U.S. House members—even though Republicans outnumber Democrats overall in both chambers. And this elite-level gender gap is certain to grow after the 2018 midterms. Democrats have nominated 183 women for the House this year (compared to 52 for the Republicans), representing a record 43 percent of the party's candidates. Among non-incumbents, a full 50 percent of Democratic House candidates are female, compared to 18 percent for the GOP.