Donald Trump has been the center of attention since the first Republican presidential debate last week. But perhaps the most significant policy moment in the debates came when two other GOP frontrunners, Florida senator Marco Rubio and Wisconsin governor Scott Walker, announced their opposition to abortion without any exceptions.

During the Cleveland debate, moderator Megyn Kelly asked Rubio to clarify whether he supports exceptions on abortion, including for rape or incest. “I have never said that. And I have never advocated that,” the Florida senator responded. He reiterated his opposition to abortion exceptions over the weekend.

Not to be outdone, Walker declared in the debate that he, too, does not support any exceptions to abortion. “Would you really let a mother die, rather than have an abortion?” Kelly asked. Walked responded that he’s “always been pro-life” and has “said many a time that that unborn child can be protected, and there are many other alternatives that can also protect the life of that mother”. (When a Republican congressman made a similar claim in 2012, the American College of Obstetricians and Gynecologists put out a statement that “abortions are necessary in a number of circumstances to save the life of a woman or to preserve her health”.)

For decades, Republicans have been the pro-life party on abortion. But for the party’s presidential nominee, that pro-life position has included exceptions.

What makes these statements from Rubio and Walker notable is that, if either man wins the primary, he could be the first Republican presidential nominee in history to openly oppose abortion in all cases.

“I am pro-life and believe that abortion should be limited to only instances of rape, incest or to save the life of the mother,” the 2012 GOP nominee Mitt Romney said in a National Review op-ed. Same for 2008 nominee John McCain, who said: “My position has always been: exceptions of rape, incest and the life of the mother.” Likewise with George W Bush, the party’s nominee in 2000 and 2004, who declared: “My position has always been three exceptions: rape, incest and the life of the mother.”

The party’s 20th century nominees all supported abortion exceptions as well. Even Republican hero Ronald Reagan supported not only allowing abortions when the life of the mother was at stake, but providing federal funding for that as well. Years earlier, Republicans nominated Barry Goldwater, who was pro-choice, and Dwight Eisenhower, who went on to serve as honorary co-chair of Planned Parenthood after he left the presidency. Prior to the 1950s, abortion was not widely considered a political issue and few politicians commented on the matter.

This isn’t surprising to Kristin Luker, a professor of law and sociology at the University of California, Berkeley, and author of Abortion and the Politics of Motherhood. “The Republican party was historically the party of sexual liberals,” she said. In the mid-20th century, Luker noted, the party’s demographic base was “well-educated, affluent people” who “had a vested interest in access to contraception”.

George HW Bush was such a champion of contraception during his time in Congress that he earned the nickname “Rubbers”. Even Reagan, as governor of California in the 1960s and 70s, signed one of the most liberal abortion laws in the country.

However, that previous support began to erode in the early 1970s. According to Luker, “Richard Nixon – never one to pass up an opportunity – figured that by opposing abortion he could carve off Catholics”, who had been a traditional Democratic constituency, for the GOP. This move was a part of Nixon’s infamous “southern strategy”, which not only won him re-election in a landslide, but also set the Republican party on a path of increasingly embracing social conservatism.

It wasn’t until around 1980, Luker said, that “abortion really became a litmus test” for the GOP. In part because of civil rights and in part because of abortion, she noted, “well-off intellectuals began migrating to the Democratic party”, creating a feedback loop that drove Republicans further to the right on reproductive issues.

So is Walker and Rubio’s no-exceptions position just a natural result of the GOP’s evolution on abortion? Not necessarily. Even former Arkansas governor Mike Huckabee and Texas senator Ted Cruz, both of whom are playing for the party’s rightwing voters, have supported an abortion exception for the life of the mother.

Though Walker and Rubio are trying to stand out in a crowded field, especially to religious conservatives in Iowa, opposing all exceptions doesn’t just put them out of step with recent GOP nominees. It puts them at odds with most Republican voters as well. A 2014 Quinnipiac poll found that just one in six Republican voters supported banning abortion in all cases, the same proportion of Republicans who believe abortion should be legal in all cases. According to Morris Fiorina, political science professor at Stanford University and author of Culture War? The Myth of a Polarized America, opposing all exceptions to abortion “is extreme even by Republican voter standards” and “only a small minority would go so far as Walker and Rubio”.

Opposition to all abortions is an even more unpopular position in the general election. A new poll this week on behalf of Planned Parenthood found that just 16% of voters in Ohio, 11% of voters in Pennsylvania, and 8% of voters in New Hampshire supported banning all abortions.

If either candidate does succeed in winning the nomination, political director at Naral Pro-Choice America Erika West predicts they will try to walk back their position. Support for banning all abortions “makes them completely unelectable”, West said. “When it’s time to run in the general, I am 100% certain we will hear something different.”

Indeed, both Rubio and Walker have indicated support for some abortion rights in the past. Walker declared in a 2014 ad that he is “pro-life” but supports legislation that “leaves a final decision to a woman and her doctor”. Rubio, meanwhile, co-sponsored a 2013 anti-abortion measure that included exceptions for rape, incest and the life of the mother.

If either candidate succeeds in becoming the first GOP nominee to oppose abortion in all cases, it will represent a significant rightward shift for the party, and one that Democrats will surely try to use to their advantage next fall.