WASHINGTON, D.C. — The U.S. Senate began debating a climate change bill this week that many view as having little chance of passing.

President George W. Bush warned Monday he would veto the Lieberman-Warner Climate Security Act in its current form because he believes its $6 trillion price tag will burden the economy. Proponents argue the cost of inaction is greater and for the need for the U.S. to take a leadership position.

"This bill will signal that the United States, after a long period of doing nothing, is prepared to stand up tall and to lead," Sen. Dianne Feinstein, a California Democrat, told the session, Reuters reported.

Many view the bill as a dress rehearsal for a new presidential administration because all presidential candidates favor a mandatory reduction in greenhouse gas emissions, although Sen. John McCain (R-Ariz.) recently signaled his belief that the bill doesn’t do enough to promote nuclear energy. The Democratic candidates would both like to see emissions allowances auctioned off, rather than being partially allocated under the Climate Security Act.

“If we get a majority or even close, we will be that much closer to addressing global warming and passing legislation in the next Congress, whoever the president is,” David Doniger, director of climate policy at the Natural Resources Defense Council, told the New York Times. “We may not get it done this year, but if not we start next year just a few steps from the finish line.”

The bill calls for cuts in greenhouse gas emissions by two-thirds below 2005 levels by 2050, although many scientists say emissions must be cut by 80 percent below 2000 levels to avoid the most extreme impacts of climate change.

The legislation’s key engine for reducing the emissions is a cap-and-trade scheme where companies would pay to pollute and be rewarded for reducing emissions. It would cover roughly 87 percent of the country’s emissions beginning in 2012.

The bill also would include billions in funding to help cover higher energy costs for the poor, efficiency projects and clean tech research and development that could spur the creation of millions of green collar jobs.