For example, the A.N.E.S. asked people if they favored or opposed the legality of abortion if staying pregnant would cause a woman to die. Across the parties, overwhelming majorities (nearly 70 percent of Republicans and 80 percent of Democrats) agreed that abortion should be legal in this situation.

Another question was whether abortion should be legal if the child were not going to be the sex the woman wanted it to be. Across the parties there was a clear pattern: Hardly anyone in the United States (only 5 percent of Republicans and 10 percent of Democrats) believed abortion should be legal in this case.

The legality of choosing to end a pregnancy after a rape was also supported by a majority of voters on both sides. And having an abortion for financial considerations found less than majority support across both parties.

It appears as if the giant party difference in support for abortion under any and all conditions is an artifact of the way the typical abortion survey question is constructed. Most respondents are not thinking about any or all possible circumstances when they answer the question; after all, you cannot think of a circumstance that you don’t know is even possible or likely. But when the A.N.E.S. provided these circumstances to people specifically, like an abortion for a woman who wanted a boy but found out she was having a girl, hardly anyone thought it should be legal to behave this way.

The data permit an even closer look at the 57 percent of Democrats who responded that abortion should “always be legal.” Clearly, some of them changed their minds when posed the question about a baby’s sex leading to an abortion. Among these “always legal” Democrats, 65 percent subsequently responded that abortion should not be legal if the woman does not like the sex of the child. Similarly, 23 percent would not support legal abortion in situations of financial hardship.

The same patterns can be found among Republicans who said that abortion should “never be legal” — 22 percent of them would support legal abortion if carrying the baby to term could result in a woman’s loss of life, for example.

Many people may find the points of agreement across party lines on the topic of legal abortion surprising. The data show that in some circumstances in which a woman may choose to end a pregnancy, majorities of Americans — Democrats and Republicans — are on the same side, sometimes supporting the legality of the procedure and sometimes not.

Focusing on the exact details of abortion decisions may reveal more about when Americans agree on this difficult issue than when they disagree.