President Obama's climate-change legislation begins a daunting march through the Senate this week, with supporters acknowledging they are as many as 15 votes shy of victory and well aware that deals to attract more votes could erode the bill's environment-friendly objectives. Senators will weigh a slew of potential compromises -- everything from allowing more offshore drilling for oil and natural gas to increasing funding for nuclear energy -- that they think would inch the package closer to passage. But environmental activists warn that the 1,400-page House version of the bill already includes so many giveaways to corporate America, that more horse-trading in the Senate could lead them to oppose the final version. Four of Obama's Cabinet secretaries will kick-start the push for the climate bill when they appear Tuesday before the Senate's Environment and Public Works Committee, which is drafting its portion of the climate legislation. Obama will be promoting the effort to limit greenhouse gases at the Group of Eight meeting in Italy on Thursday, when the leaders of the world's largest economies are slated to focus on efforts to slow global environmental change. But the political realities of the Senate, where supermajorities of at least 60 votes are needed for practically any major piece of legislation, are weighing heavily on the chamber's leaders as they push to pass some version of the bill before the end of the year. 'As a legislator, everything is negotiable,' Senate Majority Leader Harry Reid, D-Nev., said in an interview Monday. Reid has tasked a handful of committee chairs with completing their portions of the legislation by Sept. 18, at which point he hopes to cobble together the pieces and get the package to the floor late in the fall. As outlined, the bill would create a 'cap-and-trade' system placing the first national limit on greenhouse-gas emissions, gradually tightening those limits over the next four decades with a goal of reducing emissions 83 percent by 2050. Major emitters of greenhouse gases -- including any business that burns fossil fuels such as oil, natural gas or coal -- would have to reduce their emissions or buy allowances, which would be traded on markets like commodities. As of today, Reid can count on the support of about 40 to 45 senators for that basic premise, according to aides and outside activists backing the legislation. Supporters are targeting a pool of roughly two dozen lawmakers -- including about 15 of Reid's Democrats -- who will determine the legislation's fate. The battle ahead differs from many on Capitol Hill in that ideology is considered to be less influential than geography. Even some of the chamber's most liberal members have resisted signing on as they await the best deal possible for key industries in their states. Democrats from the Rust Belt states of West Virginia, Ohio, Indiana and Michigan are pushing for more incentives to help their depressed industries shift to alternative energy sources. The same senators also will likely want more funding for carbon capture and sequestration, a controversial and still-evolving technology described by its developers as 'clean coal' but derided by many environmentalists. The technology is already slated for $10 billion in government-funded research in legislation that passed the House. A trio of Democrats from the Dakotas want more funding for wind power. Sen. Byron Dorgan, D-N.D. won approval in the energy committee last month for the inclusion of new exploration for oil and natural gas as close as 45 miles off of Florida's coast on the Gulf of Mexico. That measure might help attract moderate Democrats and some Republicans, but it would almost certainly lose the vote of Sen. Bill Nelson, D-Fla., who has regularly vowed to help filibuster any bill that brings drilling within the current limit of 125 miles. Even after making additional compromises to win over wavering Democrats, Reid could find himself a few votes short and desperately searching for Republican support. Maine's moderate Republicans, Sens. Susan Collins and Olympia Snowe are the only likely GOP backers of the legislation at this point, and if Obama needs more Republicans, he may have to authorize Reid to give in for more funding for the construction of the nation's first new nuclear power plants in a generation. The environmental lobby has rigorously opposed any new nuclear plants but several GOP senators, including Lamar Alexander (Tenn.) and John McCain (Ariz.), have made their case that nuclear power is the best for cleaning the skies of carbon emissions. The narrow 219-213 victory on June 26 in the House has given Senate backers some level of hope, despite the concessions they might be forced into accepting. 'I am every optimistic. As a legislator and a chairman, I don't deal in hypotheticals, I don't think negatively. I think positively, especially after the House vote,' Sen. Barbara Boxer, D-Calif., chairman of the environment committee, said Monday. Some outside activists supporting the bill are taking a wait-and-see approach, acknowledging that they cannot let the perfect be the enemy of the good. 'Senate leaders will likely be compelled to expand the political and policy appeal of the bill to reach key moderates in both parties,' said Paul Bledsoe, director of communications and strategy for the bipartisan National Commission on Energy Policy. But others question the worthiness of legislation designed to reduce the effect of the oil-and-coal-drive manufacturing sectors if it includes giving more breaks to just those industries. 'It goes a little in all directions,' said Anna Aurilio, director of the Washington office for Environment America. Staff writer David A. Fahrenthold contributed to this report. 