The rising number of women in Congress can obscure another trend: The number of Republican women has remained roughly stagnant for more than a decade.

Although women in both parties have increased their numbers in Congress during the past 25 years, the share of Democratic women — now nearly 33 percent — has continued to climb, while the Republican female share has leveled off since hitting 10 percent during the mid-2000s. And political polarization seems to be a major reason.

Moderate Republican women — think of Olympia Snowe, the former Maine senator, or Connie Morella, the former Maryland congresswoman — were once common in the party, according to research by Danielle Thomsen, a political scientist at Duke. But moderate Republicans of both genders are nearly gone from Congress today.

Some conservative women, like Sen. Joni Ernst of Iowa, have been elected, but there are relatively few of them in a traditional pipeline to Congress: state legislatures. In other words, the gap is likely to persist for some time.

As one measure of the gap, 17 Republican women have served in the Senate in its history, and 14 Democratic women currently serve in the Senate.

The pattern is part of a larger gender disparity in American politics, of course. In 2012, 55 percent of women voted for President Barack Obama, while 52 percent of men voted for Mitt Romney, according to exit polls. The gap figures to continue, if not widen, in 2016, with Democrats seeming likely to nominate a woman for president and Republicans likely to nominate a man.

There is plenty of research that the presence of women in legislative bodies makes a difference, particularly on the policies that many female lawmakers prioritize, such as health care and children’s issues. Interviews with women in Congress by the Center for American Women and Politics have found that many see themselves as "surrogate representatives" for women in general.

A root cause of the gap is that Democratic women who are potential congressional candidates tend to fit comfortably with the liberal ideology of their party’s primary voters, while many potential female Republican candidates do not adhere to the conservative ideology of their primary voters.

But it is not just moderate Republican women who have been affected by the polarization of the parties. "If you look at the moderate men, they’re not there, either," Thomsen said. Conservative Democrats in state legislatures also are not running for Congress.

Thomsen found that 1 in 5 Republican state legislators of either sex could be described as moderate, based on their voting history and donors, but moderates were not nearly as eager as conservatives to run for Congress.

"Conservative Republican men and women state legislators are equally likely to run for Congress, but women are outnumbered 5-1," Thomsen said.

Separate research, by Shauna Shames, a political scientist at Rutgers University, suggests that women with fiscally conservative and socially liberal views particularly struggle to find a party home. "A whole lot of women who could run are not running, because they know they will not be supported and don’t feel that they fit in the party anymore," Shames said.

The Republican women who have run in congressional primaries over the past 25 years have been as conservative as Republican men, according to a study produced this year by Political Parity, a program that pushes for more women in Congress. (Thomsen and Shames both worked on the study). There simply have not been very many highly conservative female candidates, compared with men.

On the Democratic side, the situation is different. Female candidates for Congress were more liberal on average than their male counterparts, the study found, helping them do well in party primaries, which emphasize ideological purity.

Thomsen’s research does suggest that the number of conservative women in Congress will increase in coming years, because the number of conservative women in state legislatures has recently risen, albeit slowly. The three most recently elected female Republican senators — Ernst, Shelley Moore Capito of West Virginia and Deb Fischer of Nebraska — are all conservative and all served in their state legislatures.

Yet the share of congressional Democrats who are female may also continue rising, which could keep the current gap from shrinking.

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