This represents a major "sea change" in the attitudes of Americans in the pro-life / pro-choice debate. The poll was conducted May 7-10, finding 51% of Americans calling themselves "pro-life" on the issue of abortion and 42% "pro-choice."
A year ago, 50% were pro-choice and 44% pro-life. Prior to now, the highest percentage identifying as pro-life was 46%, in both August 2001 and May 2002.
Meanwhile, when asked if abortion should be legal under "certain circumstances," "any circusmtances," or "no circumstances," about as many Americans now say the procedure should always be illegal (23%) as say it should always be legal (22%). The number who say it should be legal uncer certain circumstances is 53%. During the past four years, "always legal" has been significantly above "always illegal" in ranking.
So, the vast majority of Americans still believe abortion should remain legal, at least under "certain circumstances." When you add those groups together, you have 75% who believe it should still be legal.
It is, however, interesting to note that among the group who want abortion to be legal only under "certain circumstances," the poll found that the majority of those folks want abortion really to be legal "only in a few circumstances."
I would tend to believe those would mostly be in the "danger to mother," rape, and those types of circumstances, although there was no elaboration by Gallup.
In terms of why, Gallup believes it is because of President Barack Obama's pro-choice leanings:
It is possible that, through his abortion policies, Obama has pushed the public's understanding of what it means to be "pro-choice" slightly to the left, politically. While Democrats may support that, as they generally support everything Obama is doing as president, it may be driving others in the opposite direction.