Support for abortion rights depends on how you ask the question
Justice Anthony Kennedy's resignation from the Supreme Court and President Trump's nomination of Brett Kavanaugh have created the possibility of a more conservative Supreme Court overturning the landmark 1973 Roe v Wade decision, reopening the issue of abortion in American politics.
Multiple polls have been released in the past few weeks showing low support for overturning Roe v Wade -- a good sign for Democrats and some Republicans who support it. However, abortion isn't necessarily the clear cut topic that it appears to be from those numbers. Every poll will show you a different number, complicated by gray areas over what kind of abortion people feel should be legal and the large influence of polling question wording and how abortion (a sensitive topic to begin with) is framed.
These factors have a lot to do with how people view abortion in any given poll.
All polling used below is from Gallup. Due to question wording and methodological differences, using one pollster to compare numbers is the best way to ensure consistency.
There are a few ways to frame the number of people who say abortion should be legal or illegal. Technically, in the most recent Gallup poll, conducted in late May, 79% think that abortion should be legal under any or certain circumstances. However, out of that, a plurality are saying it should only be legal in a few circumstances (50%), with three-in-10 saying it should be legal in any. Only 18% said abortion should be illegal in all circumstances.
While the big number is 79% saying some abortions should be legal, there are shades of gray inside of that. It includes people who are saying women should be allowed to have an abortion only when the pregnancy would endanger her own life, which would presumably be included for a person who favors "only certain circumstances." The entirety of the 79% probably doesn't agree on all other aspects of the debate surrounding abortion.
The total number who say abortion should be legal in some or all circumstances hasn't risen significantly since it was originally tested in 1975. In fact, in April 1975, 75% said abortion should be legal. The plurality has always been among those who say abortion should be legal under certain circumstances, followed by legal in all circumstances and illegal in all circumstances battling it out for second, depending on the year.
It's true that those who said abortion should be legal in all circumstances has increased since 1975. However, the 29% who say abortion should be legal in all circumstances isn't the highest it's ever been in the most recent polling. The high came in July 1992, when 34% of Americans said it should be legal in all circumstances.
Currently, states can regulate abortion but the test is whether the regulation imposes an "undue burden" on a woman seeking abortion and creates a substantial obstacle "to the procedure before the fetus develops to the point of viability." That test was established in the 1992 Planned Parenthood vs. Casey decision. States regulate abortion differently up to that point.
Roe v Wade
Roe v Wade has relatively high support in polls, with more defined support than for a general abortion legalization question.
In July 2018, 64% said they don't want Roe v Wade overturned, an 11-point change from Gallup's last poll in 2012. That's a six-year gap, but having the issue back in the media tends to ignite some urgency for people to have an opinion, one way or another, with the number who say they have no opinion cut in half (from 18% to 9% since 2012).
The landmark decision, passed in 1973, made abortion legal nationwide. There has always been a majority who want to keep Roe v Wade since Gallup began polling on the topic in 1989. But t's hard to compare support for legalized abortion to support for keeping or overturning Roe v. Wade since it's comparing a black-and-white choice versus shades of gray and the question wording seems to influence the answers.
Pro-life or pro-choice
Another way to frame the abortion debate is whether people self-identify as "pro-life" -- people who don't support abortion rights -- or "pro-choice" -- people who do support abortion rights. Here the polling tilts away from support for abortion rights and toward being "pro-life."
There is support for legalized abortion and for not overturning Roe v. Wade, but using the shorthand that has been used by advocates on the issue creates an even split. In Gallup's most recent poll on the topic in May, 48% of Americans said they're "pro-choice," and the exact same amount saying they're "pro-life."
That May poll reflects the highest percentage of people saying they're "pro-life" since 2013. Those who said they were "pro-life" dropped between 2013 and 2014, and has moved back up since then.
More people consistently said they were "pro-choice" than "pro-life" from 1995 through 2009, with one point in 2001 when the two terms were tied. Since then, the two have traded back and forth with one slightly above the other for the American public every few years.
The terms "pro-life" and "pro-choice" are perfect examples of framing political opinions in their best possible light. No one wants to claim they're against life or choice. CNN generally uses phrases such as supporting or opposing "abortion rights." The terminology is an explanation for why this question tends to be more closely divided than other abortion questions.