By Doug Palmer

WASHINGTON, Feb 29 (Reuters) - Democratic presidential candidates Hillary Clinton and Barack Obama, in a pair of newly released statements, have said they want stronger labor provisions in world trade agreements -- a demand long resisted by developing countries who fear it could backfire on them.

The candidates' stance raises a potential new problem for the six-year-old Doha round trade talks, which European Union Trade Commissioner Peter Mandelson already warned on Friday faced a "high risk of failure."

Developing countries have opposed including labor provisions in world trade agreements. They fear such provisions could be used by rich countries as an excuse to block imports.

"The labor issue has always been very controversial" in the World Trade Organization, said Ed Gresser, director of the Progressive Policy Institute's trade and global markets project.

"That's one of the reasons there isn't a labor mandate in the Doha round. (Developing countries) have always resisted it," he said.

Former President Bill Clinton caused an uproar at the World Trade Organization's failed 1999 meeting in Seattle when he said labor provisions should be included in trade pacts.

Now his wife, New York Sen. Clinton, and Illinois Sen. Barack are in a tight race for the Democratic presidential nomination, which could potentially be decided in a pair of primary contests on Tuesday in Texas and Ohio.

With many Ohio voters blaming trade agreements for their state's manufacturing job losses, a grass roots coalition of labor and other environmental groups asked the two candidates to more fully explain their views on the Doha round and other pressing trade issues.

Both candidates, in responses released on Thursday by the Ohio Conference on Fair Trade, said they wanted strong, enforceable labor provisions as part of the WTO.

"If trade is to work, it cannot just benefit elites. It must benefit workers as well, and it must elevate standards of living here and around the world. That is why strong, enforceable labor and environmental standards are so important," Clinton said.

Clinton also repeated negotiating new trade agreements would not be a priority if elected president.

"Rather, it will be to review our existing trade agreements to assess whether they are delivering benefits and to formulate a comprehensive trade policy that is genuinely pro-worker and pro-American," she said.

Obama said he also planned to review existing U.S. trade deal and would "work to ensure all trade agreements contain strong and enforceable, labor, environmental and health and safety standards.

"I believe that we can work within the framework of the WTO to ensure our international standards for workers, poor nations, public health and environment are all improved," he said. (Editing by Alan Elsner)

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