There are two basic branches of the pro-life movement. They both believe abortion is wrong, but one group—let's call them the "incrementalists"— believe the best way to end abortion is to gradually chip away at the availability of abortion until the procedure is so hard to obtain that it practically never happens. This wing is responsible for measures like Texas's HB2, which mandated that abortion providers gain hospital admitting privileges and meet the standards of ambulatory surgical centers. They're behind most of the abortion restrictions we've seen in the past couple of years, like forcing women to look at sonograms of their fetuses before they terminate their pregnancies, 20-week abortion bans, and limitations on medication abortion.

The other branch, let's call them "absolutists," believe the best way to stop abortion is to, well, stop abortion. That means in all cases, preferably with an across-the-board law. Personhood measures fall in this category because they would mean that "unborn human beings" meet the legal definition of a "child." This wing is motivated by the fact that no matter how many incremental restrictions are passed, there will still be some women somewhere who will manage to follow all the rules and get an abortion.

“Step-by-step measures haven’t stopped the killing,” Linda J. Theis, president of an absolutist group that broke away from Ohio Right to Life, told The New York Times in 2011.

The absolutists haven't been doing as well, though, because most voters are relatively moderate on abortion. The electorate has been almost evenly split between the "pro-life" and "pro-choice" camps since about 2000. Even if they oppose abortion in theory, most people would probably support the right of rape victims to get one—something this most recent Colorado measure would have outlawed.

What's more, personhood laws are written using very vague language—often purposefully so—so people aren't quite sure what their effect would be. North Dakota's Measure 1, for example, would have added a paragraph to the state constitution that said: "the inalienable right to life of every human being at any stage of development must be recognized and protected."

"The impact was uncertain and the thing itself was uncertain," said Mary Ziegler, a law professor at Florida State University who has studied the abortion debate. "Voters just aren’t comfortable with restrictions that are that absolute."

The incremental approaches have proven far more popular. Tennessee, for example, passed a measure Tuesday that gives the General Assembly more flexibility to legislate abortion laws, as my colleague Emma Green wrote.

The issue facing the absolutists, Ziegler says, is that "voters are not comfortable with abortion, and they aren't comfortable with banning it."

Tuesday's election might signal the twilight of such statewide personhood ballot initiatives, if only because these defeats come at a huge political cost: They're demoralizing to activists, and they make politicians think they don't have to pay attention to pro-life voters, Ziegler says.