Mitt Romney signed the National Organization for Marriage's five-point pledge that promises support for a federal constitutional amendment "defining marriage as the union of one man and one woman."

"Rep. Michele Bachmann and Rick Santorum also signed the pledge, though that's not much of a surprise: both candidates have grounded their campaigns in part in strong social conservatism," CBS News reports. "Romney's decision is more interesting because he has largely tried to keep his focus on fiscal issues and declined to sign an earlier, more far-reaching 'values' pledge from a conservative group in Iowa."

Minnesota's former Gov. Tim Pawlenty is one prominent Republican presidential candidate who won't be signing the marriage pledge.

Pawlenty, "who has sought to burnish his credentials as an evangelical Christian, declined to sign it, possibly putting him at odds with the social conservatives he is trying to court in Iowa," a Wall Street Journal article details. The National Organization for Marriage is running a four-day Iowa bus tour leading up to the Aug. 13 straw poll in Ames.

Last month Romney declined to sign a pledge denouncing same-sex marriage from the conservative group Family Leader — a document that included 14 separate points.

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