Some of the nation's most divisive social issues - gay marriage, abortion and affirmative action - went before voters today as 36 states passed judgment on ballot measures as well as candidates.

Of the 153 measures at stake, the most momentous was a proposed constitutional amendment in California that would limit marriage to heterosexual couples.

Similar measures have prevailed previously in 27 states, but none were in California's situation - with thousands of gay couples already married in the aftermath of a state Supreme Court ruling in May.

South Dakota's ballot included an initiative that would ban abortion except in cases of rape, incest and serious health threat to the mother. A tougher law without the rape and incest exceptions was defeated in 2006; a recent poll on the new version suggested the outcome was too close to call.

If it passed, it would likely trigger a legal challenge which could lead to the U.S. Supreme Court and a reconsideration of the 1973 Roe v. Wade ruling. Colorado has an amendment that would define human life as beginning at conception. It doesn't explicitly mention abortion, but activists on both sides in the campaign view it as a direct challenge to abortion rights.

Two other measures also have drawn the interest of the rival sides in the abortion debate - a California proposition that would require parental notification for a minor's abortion and a Michigan initiative that would loosen restrictions on stem cell research.