"In some ways, this is a dress rehearsal for next year, but I still think it will be a useful thing for the Senate and Congress, because at some point we have to deal with it," said Sen. Byron Dorgan, a North Dakota Democrat, who has yet to decide how he will vote.
For the moment, supporters of establishing a federal cap-and-trade system to curb emissions linked to global warming say they hope to put down a marker in the national debate over climate change. And lawmakers from both parties are eyeing how their votes might become fodder in this fall's presidential and senatorial elections.
The bill - which would cut U.S. emissions 18 percent below 2005 levels by 2020 and slash them nearly 70 percent by mid-century - has picked up support in recent weeks from 13 unions in the AFL-CIO's building and construction trades department, the U.S. Conference of Mayors and many faith groups. It is also backed by companies such as General Electric and Alcoa and utilities such as Exelon, PG&E, FPL Group and Public Service Enterprise Group.
But it has run into opposition from some energy titans who say they favor a cap-and-trade system for greenhouse gas emissions but argue that this version is the wrong one and will cost consumers too much.
"This is just a money grab," said James Rogers, the chief executive of Duke Energy. Rogers says he supports a cap-and-trade system but argues that this bill raises too much revenue from coal users while diverting too much of it to other purposes. "Only the mafia could create an organization that would skim money off the top the way this legislation would skim money off the top," he said. Duke, with customers in Ohio, Indiana and the Carolinas, relies heavily on coal-fired plants.
More than a dozen key senators - including freshmen Democrats Sherrod Brown of Ohio, Claire McCaskill of Missouri and Jon Tester of Montana - have yet to endorse the bill.
"Generally, I believe that global warming is a serious issue and that we need to address it," said Dorgan, whose state produces lignite coal as well as wind power. But he added that he is still "digesting" the complicated bill, which he fears would not do enough to spur technology that would enable the country to continue burning coal.
"We thought and hoped we'd be in a more serious place, but most people are using it as an opportunity to vet ideas and advance ideas for the debate to come in the next Congress," said Tim Profeta, who directs Duke University's Nicholas Institute for Environmental Policy Solutions. "Not many people see this as a serious piece of legislation that will become law this year."
That doesn't mean a lot of work hasn't gone into the bill. Senate Environment and Public Works Committee Chairman Barbara Boxer, a California Democrat, who has led the fight for the bipartisan bill by Sens. Joseph Lieberman, an independent from Connecticut, and John Warner, a Virginia Republican, said in a statement that it is understandable that she and her colleagues are encountering resistance.
"This is landmark legislation, and enacting landmark legislation is never an easy task," Boxer said. "There is always an excuse not to act - but in this case, the longer we wait, the harder it gets to solve this problem. Time is our enemy, and every expert has told us we face dangerous consequences from unchecked global warming if we do not address this problem now."
Still, most of the advocates who have spent years pushing for climate legislation said they hope that this week the Lieberman-Warner bill will get more than the 38 ayes a similar bill got in 2005 and the 43 received in 2003. A coal industry source said their lobbyists counted 45 senators favoring the bill and 47 definitely or leaning against it.
"The question is: Are you building for the future, or are you sending a signal this is just too hard to do?" asked Steve Cochran, who directs the Environmental Defense Fund's national climate campaign. "As long as we get our electricity where we get it from, our gas where we get it from, the same interests are at play, whether you have Democrats or Republicans in charge of Congress and whether you have a Democrat or Republican in the White House."
Some of the bill's strengths - the most detailed framework yet for how to distribute pollution allowances to emitters, how to determine what offsets polluters can buy and how to contain potential spikes in carbon prices - have sparked opposition from industry interests as well as politicians concerned about the bill's ultimate price tag.
Duke Energy has urged its commercial and industrial customers to lobby senators. One is Nucor, a steel company with plants in Indiana and other states whose chief executive is also on Duke Energy's board of directors. The American Coalition for Clean Coal Electricity, a coal industry group, has been running ads in a dozen states with senators believed to be undecided. One ad warns that "we may have to say goodbye to the American way of life we all know and love."
Political and personal differences, as well as policy ones, remain. Some Democrats, according to both Senate aides and environmental activists, resent having to vote for anything authored by Lieberman in light of his active support of the presidential bid of GOP Sen. John McCain of Arizona. And several senators are questioning why they are being asked to vote on a lengthy substitute version of the bill that Boxer and her allies just introduced a week and a half ago.
All of this has left opponents of the bill, such as Environment and Public Works ranking member and global warming skeptic James Inhofe, an Oklahoma Republican, gloating. Andrew Wheeler, the panel's GOP staff director, said in an interview that Republicans will not filibuster the bill because they relish the chance to offer amendments highlighting the bill's effect on energy costs.
"People are looking at this; they're seeing that it's going to do destructive things to energy prices and gasoline prices," Wheeler said.