While about half of American voters identify themselves as as antiabortion, a full 75 percent thought abortion should be legal in cases of rape or incest, according to a 2011 Gallup poll. Only 20 percent of Americans believed abortion should be illegal in all situations, that poll showed.
In certain demographics, however, the number stands much higher. Razib Khan at Discover Magazine uses the General Society Survey to dig into data on what populations do not support legal abortion in the case of rape. He charts the results here (keep in mind the scale goes from zero to 50 percent, not zero to 100 percent).
The highest rates of support are among the demographics you'd probably expect: those who identify as extremely conservative and those who view the Bible as the word of God. Even among those who identify as liberal, however, there is a small faction that shares this viewpoint. It's also interesting to note that opposition to abortion in cases of rape is higher among women then men.