There was once a larger bloc of anti-abortion-rights Democrats in Congress, but many of those members were from the South, and those seats are now largely occupied by Republicans as the Democrats’ strength has declined in that region. Also, as the Christian right made it a key GOP policy goal to reverse Roe v. Wade and limit abortions, pro-abortion-rights groups pushed the Democratic Party to make defending abortion rights more central to the party’s identity — so much so that the party inserted a provision into its 2016 platform that called for the government to overturn a ban on using federal funding to pay for abortions. The ban, called the Hyde Amendment, has been in place since the 1970s.

This leftward shift among Democrats has occurred as groups that promote women’s rights have gained more power in the party. While Republicans’ “identity politics” are based more on faith than gender, for Democrats it’s the other way around.

The number of Democratic women in Congress has surged in the last two decades, and now about a third of Democrats in Congress are female. Many of those women got support from the group Emily’s List, which supports female Democratic candidates, but only those who back abortion rights. Planned Parenthood, which provides reproductive health services, including abortions, is deeply enmeshed in Democratic politics. Its political action committee invests heavily to help the party win elections.