Republican presidential candidate Mitt Romney signed a pledge on Thursday promising to take legal measures against gay marriage.

Romney has largely sidestepped potentially divisive social issues and stuck to a message based on the economy, but the pledge represents a move to solidify his credentials as a social conservative. Michele Bachmann and Rick Santorum, rivals for the nomination who have been vocal opponents of gay marriage, also signed the pledge.

Authored by the National Organization for Marriage, the pledge commits signers to supporting a constitutional amendment narrowly defining marriage as between a man and a woman, upholding the Defense of Marriage Act and choosing for his judiciary appointments those "who will respect the original meaning of the Constitution."

The pledge is different from a previous anti-gay marriage oath, circulated by the conservative group Family Leader, that spoke favorably of how African-American families fared under slavery.

Romney spokesman Ryan Williams called the NOM pledge a "straightforward pledge that reaffirms Governor Romney's support for traditional marriage."