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Republican presidential candidate Michelle Bachmann is standing firm behind a pledge she signed Thursday that promotes marriage and social conservative values, but includes a passage that suggests black families were in better shape during slavery.

The Family Leader, an Iowa-based conservative group led by Bob Vander Plaats, issued the pledge formally called, "The Marriage Vow – A Declaration of Dependence Upon Marriage and Family."

The two-page document condemns gay marriage, abortion, pornography and infidelity. But perhaps the most controversial part can be found in the preamble comparing the state of the black family in the slave era to today.

"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 opening statement reads.

Alice Stewart, a spokeswoman for the Bachmann campaign, told FoxNews.com that the Minnesota congresswoman has no second thoughts about signing the pledge.

"She stands by the points that are outlined in the pledge," she said. "Particularly the ones for strong marriage. She's been happily married for 32 years. That's the focus of the pledge."

But some commentators have focused their criticism on the slavery passage.

"Given that families were broken up regularly for sales during slavery and that rape by masters was pretty common, this could not be more offensive," Cheryl Contee wrote on her blog, Jack and Jill Politics, which offers a middle class African-American take on politics.

"When will Republicans inquire with actual Black people whether or now we're ok with invoking slavery to score cheap political points?" she added. "It has to stop. It is the opposite of persuasive and is another reason Republicans repel. It's hard to believe that Michele Bachmann would be foolish enough to sign this pledge."

Former Pennsylvania Sen. Rick Santorum, another GOP presidential hopeful, also signed the pledge. But it's not clear whether he agrees with the slavery passage because his campaign did not respond to a request for comment.

But Santorum has invited controversy before by linking slavery to abortion. In January, he questioned why Obama is denying civil rights to fetuses by supporting abortion rights. He told the Christian Broadcasting Network that for decades, slavery allowed blacks to be treated like property. He said fetuses are denied the right to life because they are considered property.

The Family Leader did not reply to an email seeking comment. But Vander Plaats, who is known as a kingmaker in Iowa, has said that the pledge is an opportunity to fulfill the group's mission of strengthening families.

"The Family Leader views this pledge as an important component needed to inform constituents about the personal stand that each presidential candidate takes regarding marriage," he said in a statement. "We believe that the candidates' positions on core values, such as marriage, correlate directly to his/her moral stances on energy issues, sound budgeting policies, national defense, and economic policies."