"They would find a way to constructively work within the U.N. system," Yeo added.
Detractors say that the candidates are just being truthful.
"I wouldn't call it U.N. bashing; I'd call it U.N. realism," said Bolton, the former U.S. ambassador to the U.N. "I think the issue for the United States is what to do to make the U.N. more effective, and the answer to that has to lie in how it is funded."
Contributions to the U.N.'s regular budget are assessed on a scale based primarily on a country's ability to pay. Additional contributions to U.N. entities such as the children's agency UNICEF are voluntary.
The U.S. assessment is the highest — 22 percent of the total U.N. operating budget. By comparison, China pays 3 percent.
In the 2010 budget year, the U.S. provided $7.7 billion to the U.N. for its regular budget, peacekeeping and other programs, up from $6.1 billion the previous year.
House Republicans recently introduced legislation to force the U.N. to adopt a voluntary funding system. The administration opposes it and it is unlikely to become law.
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