Ben Smith at Politico blogs:

Former Gov. Mitt Romney has joined Rep. Michele Bachmann and former Sen. Rick Santorum in signing a pledge to oppose same-sex marriage on a number of specific fronts.

The three candidates signed the pledge advanced by the National Organization for Marriage, which has led national and state campaigns to limit marriage to a man and a woman. The signature of the frontrunner, Romney, is a bit of a coup for the group, as he's been careful about committing to other pledges, including a broad promise to a socially conservative Iowa group that caused trouble for other candidates.

Romney, Bachmann, and Santorum signed on to supporting a federal constitutional amendment defining marriage, to appoint federal judges who don't see a Constitutional right to same-sex marriage, and to back the Defense of Marriage Act.

They've also pledged to support offering a referendum on marriage in the District of Columbia, and to establish a "presidential commission on religious liberty" aimed at protecting the rights of marriage foes to speak out.

Bottom line:

The pledge is less meaningful for its direct consequences than for the fact that it commits the three candidates fully to the hottest front in the culture wars.