Advocacy group see them as the way to hold presidential candidates accountable to their promises if they get in office.
This is the year presidential candidates are being asked to take the pledge.
They are asked to state their opposition to abortion rights. They are pressed to pledge support for a constitutional amendment to balance the budget. They have been asked to oppose pornography, women in combat and sharia law -- all part of a "marriage vow" pledge.
Candidate pledges, an incidental part of past presidential elections, have exploded this year as advocacy groups seek to hold a future Republican president accountable.
Driven by the same anti-Washington fury that delivered scores of new Republicans to the House last year, the pledges aim to impose litmus tests on candidates and discourage them from altering positions later under political pressure.
"At a time when voters have grown skeptical about politicians and candidates who run on a certain platform only to backtrack once elected, signing a pledge is a good way to strengthen our political promises," Rick Santorum, a Republican presidential candidate and former Pennsylvania senator, wrote in an opinion column last week.
He has been one of the most agreeable pledge signers, adding his name even to compacts that rivals have abstained from because of controversial passages.
"The GOP is reading the mood of a key part of their constituency as telling them, 'Hold the line, we don't want you to compromise,' " said Donna Hoffman, a political scientist at the University of Northern Iowa. "A particular sector of the electorate believes compromise is invalid at this point; it's selling out your principles."
Santorum did not hesitate to sign the marriage vow pledge last week, which was written by an evangelical Christian group in Iowa. After public criticism, the group deleted a sentence stating that black children under slavery were more likely to be raised by both parents than "after the election of the U.S.A.'s first African-American president."
Michele Bachmann, a Minnesota congresswoman and presidential candidate, also signed the pledge, although she told Fox News that the slavery sentence, which appeared in a preamble, was "not on a document that I signed."
The pledge, two dense pages with footnotes, binds signers to try to block same-sex marriage as well as be faithful to their spouses, oppose women "in forward combat roles" and support "robust childbearing and reproduction."
Other Republican candidates refused to sign, including former Gov. Tim Pawlenty, who posted a six-minute video on YouTube describing how his faith shapes his view that marriage should be limited to "one man and one woman."
Mitt Romney's campaign was blunter. He found the pledge "undignified and inappropriate for a presidential campaign," spokeswoman Andrea Saul said.
Candidate pledges seem largely a Republican phenomenon this time, though liberal groups have promoted them in the past, including a Fight Washington Corruption pledge that MoveOn.org said about 200 congressional incumbents and candidates signed in 2010.
One candidate, former Utah Gov. Jon Hunstman refused to sign anything. He said: "I don't sign pledges -- other than the Pledge of Allegiance and a pledge to my wife."
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