WASHINGTON (Reuters) - Even before debate began on Monday on the first comprehensive climate change bill to reach the U.S. Senate floor, the White House said President George W. Bush would veto it in its current form.

Bush himself slammed the bill, saying it would cost the U.S. economy $6 trillion. His estimate drew quick denials from those who support the legislation, including Sen. Barbara Boxer, a California Democrat and longtime environmentalist.

The Bush administration has consistently opposed economy-wide measures to limit climate-warming emissions of carbon dioxide. The United States is alone among major developed countries in rejecting the carbon-capping Kyoto Protocol, which sets more stringent targets than the bill headed for Senate debate.

“I urge the Congress to be very careful about running up enormous costs for future generations of Americans,” Bush said at a White House meeting on the economy and taxes. “We’ll work with the Congress, but the idea of a huge spending bill fueled by tax increases isn’t the right way to proceed.”

He said the bill, known as the Lieberman-Warner Climate Security Act and set for its first debate in the Senate late on Monday, “would impose roughly $6 trillion of new costs on the American economy.”

White House spokeswoman Dana Perino said Bush would veto the bill if it arrived on his desk as currently drafted, but added, “It’s very unlikely to pass the Senate anyway.”

Most Capitol Hill observers acknowledge the bill is highly unlikely to become law before Bush leaves the White House in January.

FENCE-SITTERS IN THE SENATE

The number of uncommitted senators is impressive, said Jeremy Symons of the National Wildlife Federation.

President George W. Bush makes remarks after the Ford's Theatre Gala at the National Theatre in Washington, DC June 1, 2008. REUTERS/Joshua Roberts

“The fence is so crowded you can hear it creaking,” Symons said of the fence-sitters on global warming legislation.

Some estimates have put the number of uncommitted senators at up to 20 -- one-fifth of the 100-member Senate.

The bill’s supporters maintain that the legislation’s cap-and-trade provisions would create jobs and that the cost of doing nothing about climate change justifies action now.

Carbon dioxide, which is emitted by fossil-fueled vehicles and coal-fired power plants as well as from natural sources, is a powerful greenhouse gas that contributes to global warming.

Boxer, who chairs the Senate Environment and Public Works Committee and who has shepherded this legislation to the Senate floor, offered a blistering response to Bush’s comments.

“Just when we finally have a chance to get off of Big Oil and foreign oil, you can count on the Bush administration to fight us every step of the way,” she said in a statement. “Where were they when gas prices went to 250 percent of what they were at the start of this administration? They did nothing.”

The Lieberman-Warner bill contains tax relief for consumer energy costs, Boxer said.

“As you can imagine, our opposition to this will be quite strong and we’ll be making these points throughout the week,” said Keith Hennessey, director of Bush’s National Economic Council.

U.S. gross domestic product could be reduced by as much as 7 percent in the year 2050 and gasoline prices -- already at record highs in the United States -- could soar by as much as 53 cents a gallon by 2030, he said.

The legislation the Senate will debate could cut total U.S. global warming emissions by 66 percent by 2050, according to a summary of the measure.

U.S. greenhouse gas emissions would drop by about 2 percent per year between 2012 and 2050, based on 2005 emission levels, under the measure.

The bill would cap carbon emissions from 86 percent of U.S. facilities, and emissions from those would be 19 percent below current levels by 2020 and 71 percent below current levels by 2050, according to a summary of the bill’s details released by the Senate Environment and Public Works Committee.