“There were a lot of voices saying, ‘Why do this now? Why do we have to do this now?’ ” Mrs. Boxer said, opening the Democrats’ argument for the bill. “Because it is, in fact, one of the greatest challenges of our generation and we have to respond with a landmark bill, and it will take us awhile and we must get started.”

The bill is a revision of a plan proposed last year by Mr. Warner and Senator Joseph I. Lieberman, independent of Connecticut.

The measure would reduce American production of climate-altering gases by nearly 70 percent from current levels by 2050. It would provide billions of dollars in subsidies for energy conservation and environmentally clean technologies, creating millions of jobs, proponents say.

The sale of the permits would raise more than $5 trillion for the government in the coming decades, money that the bill proposes to distribute to affected industries, consumers and local governments in one of the biggest programs of redistribution of American wealth in history. The bill’s proponents say the money would help pay for a technological leap that would create millions of new jobs while cleaning the atmosphere.

Given the timing and the range of opposition, it appears unlikely that the bill will pass the Senate. (Similar efforts are under way in the House.) President Bush has stated opposition to the mandatory emissions limits and numerous other provisions.

But history shows that far-reaching environmental legislation often gets off to a slow start, then passes both houses of Congress and is signed into law when the political time is ripe. “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,” said David Doniger, director of climate policy at the Natural Resources Defense Council. “We may not get it done this year, but if not we start next year just a few steps from the finish line.”

Senator Bob Corker, a freshman Republican from Tennessee, said in an interview that he favored a cap-and-trade program in theory but found serious flaws in the bill. He will be among those seeking to derail it.