(Curtis Compton/AP)

It wasn’t always this way: as recently as the 1970s, views on abortion didn’t break down along party lines. As a number of academic studies have pointed out, abortion has been transformed from an incidental political issue to a core litmus test over the past 30 years.

From 1972 to 1980, there was no correlation between voters’ views on abortion and their political affiliation, a 2002 study in the journal Political Behavior found. That meant that a Republican could capture the presidency even with a lengthy track record of supporting abortion rights. Exhibit A for this is President Ronald Reagan. During his tenure as California’s governor in the 1960s, Regan signed some of the nation’s most permissive abortion rights legislation. This came well before Roe v. Wade, the 1973 Supreme Court case that would require every state to recognize a legal right to abortion.

But as Reagan came into the presidency, the politics of abortion rights were in flux. The Roe decision in 1973 brought more attention to the issue. Evangelical voters, who tend to oppose abortion rights, were gravitating more toward the Republican party. In 1980, both Democrats and Republicans added planks to their national party platforms taking positions on the issue. And as president, Reagan became a staunch opponent of abortion rights. “Today there is a wound in our national conscience,” the president said in his 1986 State of the Union address. “America will never be whole as long as the right to life granted by our Creator is denied to the unborn.” Since then, political scientists argue, the abortion issue has broken sharply on party lines.

To be sure, politicians do not always line up perfectly with their parties on abortion issues: there is a notable contingent of “pro-life Democrats” who oppose access to abortion. But, as far as presidential elections go, abortion has become a key dividing issue and litmus test that candidates must pass.