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 Senator 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.