* Conservative groups pressure candidates to sign pledges
* Candidates on spot over taxes, abortion, debt, marriage
* Romney won't sign on abortion; Huntsman won't sign any
By John Whitesides
WASHINGTON, July 13 (Reuters) - The Republican presidential contenders are pledging their allegiance -- to low taxes, reduced spending, healthy marriages and more.
The party's White House hopefuls are under pressure from advocacy groups seeking pledges of support on an expanding list of social and fiscal issues, from short-term vows on debt-ceiling talks to personal promises of marital fidelity.
The pledges are designed to put candidates on the spot and force them to take a stance on key issues. But critics lament their inflexibility and say they have become ideological litmus tests for an increasingly conservative party.
A few Republican White House contenders, most notably front-runner Mitt Romney, have refused to sign some of the pledges. Former U.S. ambassador to China Jon Huntsman has refused to sign any of them.
"I don't sign pledges -- other than the Pledge of Allegiance and a pledge to my wife," Huntsman tells crowds.
Huntsman preferred to put his record into his own words. "He's going to run on his record, and his record is stronger than any pledge," said his spokesman, Tim Miller.
The Republicans running for the right to challenge Democratic President Barack Obama in 2012 have been asked to sign at least five pledges, in addition to filling out the usual array of questionnaires from advocacy groups.
The oldest and most established pledge is a simple promise to oppose all tax increases, introduced a quarter-century ago by Grover Norquist and his group, Americans for Tax Reform.
All of the top Republican White House hopefuls except Huntsman have signed it, as well as 235 House and 41 Senate Republicans. That poses a significant obstacle to Obama's quest for Republican support for a deal to raise the U.S. debt limit that includes some tax increases.
"It's designed to be inflexible. That's the whole point," Norquist said of his pledge. "There is no context. There are no weasel words. You don't take the pledge if you don't plan to keep it."
ROMNEY WON'T SIGN ANTI-ABORTION PLEDGE
Romney, the former governor of Massachusetts, drew criticism from some conservatives for refusing to sign his name to an anti-abortion pledge signed by six other contenders.
Romney said he opposed abortion rights but the pledge was "overly broad and would have unintended consequences." He said it could end federal funding for many U.S. hospitals and limit a president's ability to appoint qualified individuals to federal positions.
Former pizza executive Herman Cain also refused to sign the pledge sponsored by the Susan B. Anthony List, which backs anti-abortion candidates. Cain, an abortion rights opponent, objected to the language in one section.
Romney and former Minnesota Governor Tim Pawlenty also will not sign a 14-point pro-marriage pledge sponsored by an influential Iowa Christian conservative group, The Family Leader, although they said they shared its values.
"Rather than sign onto the words chosen by others, I prefer to choose my own words," Pawlenty said. A Romney spokeswoman said he felt the pledge had references "that were undignified and inappropriate for a presidential campaign."
The Iowa pledge covers broad ground, from opposing gay marriage to pledging marital fidelity. It also generated controversy for a preamble suggesting black families were stronger during slavery.
The statement was later removed, but not until Republican contenders Michele Bachmann and Rick Santorum had signed it. Bachmann said she signed the pledge and not its preamble.
"That statement was not on a document that I signed," she said on Fox News Channel. "I just want to make it absolutely clear, I abhor slavery."
Another pledge is a "Cut, Cap and Balance" vow sponsored by the group FreedomWorks, which commits signers to oppose a deal on raising the U.S. debt limit unless it includes big spending cuts and caps along with a balanced budget amendment.
Candidates even have been asked to sign a pledge requiring them to eliminate spending deficits through a waste reduction strategy known as Lean Six Sigma. They would have to attend two days of training on the system.
Cain, Gingrich, Pawlenty and former New Mexico Governor Gary Johnson have signed that pledge. (Editing by Christopher Wilson)