Labels are powerful things, full of partisan spin and ideological consequences and not a little bit of over-the-top public relations. Thus you can be pro-life or pro-choice. Basically attitudes about abortion have remained largely stable over the decades, with one side or the other moving up or down depending on the political whims of the moment. But when you toss in labels, ah, then things get a even murkier.
So here's the survey question asked by Gallup: "With respect to the abortion issue, would you consider yourself to be pro-choice or pro-life?"
Technically I suppose you could be both, kinda liking life, but liking choice too, but that's nitpicky nonsense that gets away from the matter at hand -- how have things changed over time?
Not all that much.
Setting aside that this is a crappy way of asking the abortion question, the results have shifted up and down for each side over the many years. Looking at the data since 2001, pro-choice has hit a high of 51 percent (in 2006) and pro-life 50 percent (in 2009). If you draw a trend line, pro-life is edging up and pro-choice remains mostly the same, with a handful of various "undecided" answers decreasing. The latest Gallup numbers from May 2010 have it at 45 percent pro-choice, 47 percent pro-life, basically a statistical tie given the margin of error involved. See the Gallup numbers yourself here.
Another way to look at it over time is the graphic below from the American National Election Studies, available only through 2004. This shows the proportion of people who believe abortion should never be permitted to be -- maybe -- inching up slightly. Nothing dramatic, but it more or less fits the previous Gallup numbers. This is one of those issues that, when it comes to what people know, nearly everyone has an opinion.