(CNN) -- Republican presidential candidate Mitt Romney will not sign a controversial marriage vow from the Iowa social conservative group, The Family Leader.
"Mitt Romney strongly supports traditional marriage but he felt this pledge contained references and provisions that were undignified and inappropriate for a presidential campaign," said campaign spokeswoman Andrea Saul late Tuesday night.
Michele Bachmann and Rick Santorum, who are also vying for the Republican presidential nomination, signed "The Marriage Vow -- A Declaration of Dependence upon Marriage and Family."
They later had to clarify their support for the pledge because it contained a preamble suggesting black children born into slavery had better family structures than black children now.
The excerpt has since been removed.
The Family Leader is an important socially conservative group in the first-in-the-nation caucus state of Iowa, holding sway over the state's traditionally more conservative caucus-goers who influence the fate of presidential campaigns.
A spokesman former Sen. Santorum of Pennsylvania told CNN Monday he was "pleased" to sign the pledge, but agreed with the Leader's decision to remove the language about slavery.
"Senator Santorum was pleased to sign the Iowa Family Leader's pledge because he is committed to standing up for traditional marriage. The bigger question here is, why aren't more Republicans having the courage to stand up for the institution of marriage and signing this pledge?" Virginia Davis wrote in an e-mail. "With that said, Senator Santorum believed it was the right thing for the Iowa Family Leader to remove the language from the preamble to the pledge about slavery."
Bachmann spokeswoman Alice Stewart said Sunday "In no uncertain terms, Congresswoman Bachmann believes that slavery was horrible and economic enslavement is also horrible."
The passage causing the stir read, "Slavery had a disastrous impact on African-American families, yet sadly a child born into slavery in 1860 was more likely to be raised by his mother and father in a two-parent household than was an African-American baby born after the election of the USA's first African-American President."
The section's end note cited "The Consequences of Marriage for African Americans: A Comprehensive Literature Review" from 2005 -- before Barack Obama was elected president.
The Romney campaign, in announcing that he won't sign the pledge, also released a statement from state Rep. Renee Schulte of Cedar Rapids.
"One of the reasons I support Gov. Romney is his support for traditional marriage," Schulte's statement said. "However, I am glad he won't sign this ill-advised pledge. The Family Leader would do more to advance the issues that conservatives like Mitt and I support if they kept it simple."
The Family Leader said in a statement last week that the purpose of the pledge "is to have on record the personal convictions of each presidential candidate as it relates to the issue of marriage. The signing of the pledge will be a requirement for future endorsement" by the Iowa-based group.
The slavery excerpt was the first bullet point in the original pledge's preamble. "The Candidate Vow" begins the second page of the pledge. It is not clear whether Bachmann or Santorum read the preamble before signing the candidate vow.
The pledge requires signatories "to defend and to uphold" the institution of marriage as between one man and one woman.
Each signing candidate must also promise personal fidelity to his or her spouse and recognize that "robust childbearing and reproduction is beneficial to U.S. demographic, economic, strategic and actuarial health and security."
Other provisions include supporting the enactment of safeguards for married and unmarried military personnel from sexual harassment and the "rejection of Sharia Islam."
Pledges have become an important feature in this election cycle, forcing candidates to take a hard line stance over social and fiscal issues. CNN Senior Political Analyst David Gergen said these agreements are constraining for candidates.
"Pledges are proliferating in political campaigns and people are being asked to sign up to things that, in this case, are way out of bounds," Gergen said on CNN Sunday. "In other cases, they lock their hands so they can't act."
He pointed specifically to the "no new tax pledge," pushed by conservative crusader Grover Norquist's group, Americans for Tax Reform. Two-hundred and thirty House members and 40 senators, almost all of them Republicans, signed the pledge. Gergen said that agreement is now constrictive in the ongoing debt ceiling negotiations.
"That means they have promised while in Washington they will never raise taxes," Gergen said. "That makes it very, very difficult to try to reach and foster some sort of compromise to get us out of the debt problem."
CNN's Steve Brusk, Holly Yan and Gabriella Schwarz contributed to this report.