Democrats who control a key U.S. Senate panel said they would begin debating a climate change bill on Tuesday, despite a planned boycott by minority Republicans who are demanding more study of the issue.

Senator Barbara Boxer, the California Democrat who chairs the Environment and Public Works Committee, wants to have a bill approved by her panel before an international summit on global warming convenes in Copenhagen in December.

The wrangling over when debate can start illustrated how difficult it will be to get any bill to the Senate floor and passed into law before year end, complicating President Barack Obama's hopes that the United States will take a leading role in Copenhagen.

Saying she was attempting to address Republican concerns, Boxer told reporters Tuesday's work session would be suspended in the afternoon so experts from the Environmental Protection Agency could come before the committee to answer technical questions -- from Democrats or Republicans -- about the bill.

Boxer also said she would extend a deadline for Republicans to notify her of amendments they might pursue to the bill.

We're not going to rush this through, she said, adding she hoped Republicans return to the table.

Senator Richard Lugar, a moderate Senate Republican whose support Democrats would like to win, warned that it would not be constructive if Boxer pushed the climate bill through the environment panel during a Republican boycott.

A committee Democratic aide, who asked not to be identified, cited Senate rules saying that Tuesday's committee session could occur if at least 10 of the 19 members attend. Democrats control 12 of the seats.

Republicans had hoped to stop the work session with a boycott by all seven of their members, citing a different committee rule stating at least two Republicans must be present for the panel to conduct business.

The Obama administration and many fellow Democrats in Congress think that committee approval would further demonstrate momentum in the United States for reducing greenhouse gas emissions.

Republicans want more analysis from the Environmental Protection Agency of the economic impact of legislation that would require utilities, factories and oil refineries to cut their carbon pollution 20 percent by 2020, from 2005 levels.

Boxer has pointed to existing EPA analysis that suggested the legislation would have only a small impact on consumers.

U.N. Climate Change Secretariat head Yvo de Boer called on the United States on Monday to set a 2020 goal for cutting its greenhouse gas emissions.

But the full Senate was not expected to vote on a bill this year.

Democratic Senator John Kerry is holding talks with Republicans and moderate Democrats that could lead to a quite different measure being presented to the full Senate next year. It would likely retain core elements of the bill before Boxer's panel but could contain incentives for expanding the U.S. nuclear power industry and offshore oil drilling.

(Editing by Peter Cooney)

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