Peter Beinart makes the case. I find this part interesting and convincing:
n the 1990s, things began to change. Crime declined, welfare was radically scaled back, and affirmative action receded from the political stage, in part because of the deep support it enjoyed from such conservative bastions as corporate America and the military. But the culture war didn't end: It simply morphed from a struggle primarily about race to a struggle primarily about religion. In the 1990s, as the affirmative action, crime, and welfare debates subsided, the void was partly filled by gay marriage, an issue that pits not black against white, but secular against religiously orthodox. The impeachment of Bill Clinton was not a racial battle, but a battle over what standard of public morality would govern political behavior. Bill Clinton's legacy, noted political scientists William Galston and Elaine Kamarck, was to relieve some of the racial anxiety that white working-class voters felt about the Democratic Party but substitute for it a new moral anxiety, felt most acutely by whites who regularly attended church.
For Barack Obama, this shift has been useful. A black politician running in the midst of a racial culture war is virtually doomed. But amidst a religious culture war, being black is less of a handicap since blacks are the least secular element of the Democratic coalition. Barack Obama was more successful than John Kerry in reaching out to moderate white evangelicals in part because he struck them as more authentically Christian.
Ross notes that Obama is more repositioning his forces than ending the war. It's not like Obama isn't making moves on abortion, and I expect that Don't Ask Don't Tell will rise again, and it isn't like conservatives aren't seeing it. Plus, we're living in a time when economic concerns trump all. That isn't exactly the best environment to make a case for a constitutional ammendment banning gay marriage.