Updated

The Senate began what is expected to be a weeklong, contentious debate Monday over legislation to combat global warming by mandatory reductions in carbon dioxide and other greenhouse gases.

Senators voted 74-14 to proceed to the bill, but immediately it became clear Republican opponents were not going to make it easy. A request by Democrats to begin considering substantive changes in the bill was blocked by GOP opponents until Wednesday at the earliest.

Senate Majority Leader Harry Reid of Nevada promised a thorough debate that will probably last through the week, if not longer. He said it's clear that "global warming is real" and Congress must act.

But supporters of the bill acknowledged it will be difficult — perhaps impossible — to overcome a certain GOP filibuster threat against the legislation, meaning congressional action on global warming will probably be decided in the next Congress and by the next president.

Many of the GOP senators who voted to debate the issue have said they are opposed to the bill.

The Senate measure, which has wide Democratic and some Republican support, would cap U.S. emissions of carbon dioxide and other greenhouse gases, cutting them by 18 percent by 2020 and by two-thirds by mid-century. It would specifically target refineries, power plants, factories and transportation for 70 percent reductions and make emissions allowances available to be traded in an open market.

Sen. Barbara Boxer, D-Calif., a chief architect of the 500-page bill, said at a news conference the urgency of taking action against climate change cannot be overstated, declaring, "It's about our children, about their children, and about the planet we've inherited."

Democratic leaders were ready to spend the week, and possibly more, on the legislation. But the tone of the debate emerged in the opening hours.

Republican leader Mitch McConnell of Kentucky called it "a giant tax on virtually every aspect of the economy," and accused Democrats of being "laughably out of touch" in taking up the bill when the country is reeling from $4 a gallon gasoline and other high energy costs.

President Bush said at a White House event that the measure amounted to "a huge spending bill fueled by tax increases" and that it "would impose roughly $6 trillion in new costs on the American economy."

White House spokeswoman Dana Perino said Bush would veto the bill as it stands, but said it seems unlikely the legislation will clear the Senate anyway.

The White House maintained the carbon limits would "impose a huge new tax" while demanding "drastic emission cuts that have no chance of being realized and have every chance of hurting our economy."

The bill would raise gasoline taxes by 53 cents a gallon by 2030, said the White House statement, a position ridiculed by the bill's sponsors. "People would be thrilled to have gas prices rise only 2 cents a year," declared Sen. Joe Lieberman, a Connecticut independent and co-sponsor of the legislation.

Alluding to carbon-intensive industries, such as most utility companies, Lieberman acknowledged "real resistance from people who don't want to change" and fear the shift away from fossil fuels. Getting 60 votes to overcome a filibuster is "going to be hard," he said.

Boxer said it was "misinformation and an untruth" to suggest that the climate legislation would impose new taxes or raise gasoline prices. The bill proposes using proceeds from the sale of carbon emission allowances to funnel $800 billion in tax relief, over 40 years, to people facing higher energy costs, she said.

Sen. John Warner, R-Va., whose cosponsorship of the bill has been credited with bringing a scattering of GOP senators on board, also rejected the administration's prediction of economic chaos. "The president has the stick to bring back the throttle" and set new emission timetables if there are serious economic problems, he argued.

"The United States has to lead," Warner said at a press conference.

The bill's sponsors sought to blunt criticism over costs by including a pollution allowance trading system. The emissions allowances were projected to generate $6.7 trillion in revenue over 40 years.

The money would be used to ease the transition away from fossil fuels by spurring alternative energy technologies such as wind, solar and carbon-free nuclear power.

Environmentalists said the cost of inaction on global warming cannot be ignored.

"These costs (of doing nothing) dwarf the kind of costs that people are talking about from taking serious initiatives to do something about climate change," said Frank Ackerman, a Tufts University economist and co-author of a recent report on the potential economic impacts of global warming. He predicted huge annual economic costs by the end of this century from climate change, including water shortages in the West, rising sea level, increasingly intense hurricanes and even higher energy costs.