One of the few issues on which opinion has moved left over the last few years is same-sex marriage. In 1996, Gallup found that Americans opposed it by a 68 percent to 27 percent margin. Last May, Gallup found Americans in favor by 53 percent to 45 percent. That's a huge change in 15 years.
Other polls have shown similar movement. Pew Research reported last week that 45 percent favored same-sex marriage and 46 percent were opposed -- a dead heat. Pew polls in 2008 and 2009 found only 35 percent to 40 percent in favor.
This is an issue on which the differences between age groups are as large as any I can remember. In the May Gallup poll, 70 percent of those under age 35 favored same-sex marriage. Only 39 percent of those over 55 agreed.
So while opinion on one controversial cultural issue, abortion, has not changed much, opinion on same-sex marriage has changed vastly.
Why? One reason is probably that as people learn that friends and relatives are gay, they become more sympathetic to gay rights. We see a similar change in voters' willingness to elect openly gay candidates to Congress and other offices.
But increasing support for same-sex marriage causes problems for politicians. When two-thirds of voters were opposed, it didn't: Almost everyone opposed it. Possible exception: Barack Obama, running for state Senate in a university-dominated district in 1996.
As a candidate for U.S. senator and president, Obama said he opposed same-sex marriage. As president, he says he still does, but his opinion is "evolving."
This may reflect a split between Democratic core constituencies. Affluent liberals overwhelmingly favor same-sex marriage. But most black voters are opposed.
In a 2008 referendum in California, 70 percent of blacks voted against same-sex marriage. A same-sex marriage bill was defeated this year in Maryland after black Democratic legislators opposed it. Same-sex marriage would be legal in California and Maryland were it not for opposition by black voters.
Mainstream media reporters pepper Republican presidential candidates with questions about the issue but seldom ask Obama about it. But if it's a fair question for Republicans, it's a fair question for Democrats, as well.
The problem for Republican politicians is not that opposition to same-sex marriage antagonizes gay voters. According to exit polls in the last three presidential elections, gays and lesbians made up just 3 percent of the electorate, and they were one of the few groups that voted for John McCain in 2008 in larger numbers than had voted for George W. Bush in 2004.