In the next few months, the Senate will consider a sweeping effort to cap the greenhouse gases scientists say cause climate change.

If the legislation is signed into law, it would transform the U.S. economy and revolutionize the way energy is produce and consumed.

Getting it through Congress will be an uphill political battle, but the effort has gained momentum as more lawmakers are convinced by the mounting evidence that climate change is a real threat caused by carbon emissions from fossil fuels such as coal, oil and gas.

The effort got its biggest boost with the election of President Barack Obama, who made it a top priority for his administration.

Still, the details of a climate bill — and its ultimate fate in Congress — rest with a group of moderate lawmakers who fear that regulating emissions from fossil fuels could hurt their home state economies.

Here's the rundown:

CAP AND TRADE

A proposed federal program to put a cap on the number of tons of carbon emissions that could be produced by coal-fired power plants, oil refineries and industrial manufacturers. Polluters would have to buy allowances for each ton they intend to emit. They could also buy and sell allowances on an open financial market, along with investors and other players.

HISTORY:

The first efforts to create a climate change cap-and-trade law were forged in Kyoto, Japan in 1997, with a United Nations treaty binding nations to commit to reducing greenhouse gas emissions.

Although President Clinton signed the treaty, the Senate never ratified it, making the U.S. one of the only nations not subject to the Kyoto Protocol.

In 2003, then-Democrat Joseph Lieberman teamed with Republican John McCain to pass a cap-and-trade bill through the Senate, but the effort failed, as did another effort by the pair in 2005.

In 2008, Lieberman teamed with Virginia Republican John Warner on another iteration of cap-and-trade. That measure gained some bipartisan support, but Republicans ultimately used procedural maneuvers to kill it on the Senate floor.

Even if any of those bills had gained traction in Congress, it was unlikely that president George W. Bush would have signed it.

After all those false starts, efforts to pass a climate bill got a huge boost with the election of Obama, who as a senator had championed taking action on climate change.

As soon as he was elected, House Democratic leaders got to work on moving a cap-and-trade bill (H.R. 2454), which passed the chamber in June.

In September, Democratic Senators John Kerry and Barbara Boxer unveiled a Senate version (S. 1733). However, progress on that bill has been slowed by the Senate's focus on health care, and it will have to be approved by at least five different Senate committees.

With that schedule, it's unlikely to come to a Senate floor vote before early 2010.

KEY SUPPORTERS:

President Barack Obama

Sen. Barbara Boxer (D-Calif.)

Sen. John Kerry (D-Mass)

Rep. Edward Markey (D-Mass.)

House Speaker Nancy Pelosi (D-Calif.)

Rep. Henry Waxman (D-Calif.)

U.S. Climate Action Partnership

ARGUMENTS IN FAVOR:

* Carbon emissions are warming the planet and will lead to catastrophic global warming in the coming decades with consequences such as increased drought, floods, disease, starvation and political unrest. Doing something to cap the emissions is a moral imperative.

* The U.S. economy must transition away from dependence on oil, not only because it contributes to global warming, but also because it makes us dependent on foreign countries for a dwindling resource.

* Creating a carbon emissions cap will create economic incentive to invest in a wide array of clean, new energy technology resources, creating new domestic industries and jobs.

KEY OPPONENTS:

Rep. Joe Barton (R-Texas)

Rep. John Boehner (R-Ohio)

Sen. James Inhofe (R-Okla.)

Sen. Mitch McConnell (R-Ky.)

U.S. Chamber of Commerce

National Association of Manufacturers

ARGUMENTS AGAINST:

* There is not enough scientific evidence that carbon emissions are warming the planet.

* Putting a price on fossil fuel emissions would raise prices of all traditional forms of energy, from gasoline to coal-fired electricity. That cost would be passed along to consumers, hurting the economy.

* Even if fossil fuel emissions are warming the planet, the economy-wide "cap-and-trade" mechanism is too vast, complex, unwieldy and prone to being gamed. Emissions could instead be cut by capping just one sector of the economy, such as electric utilities, and ramping up production of nuclear power, which produces no carbon emissions.

KEY VOTES

The House version of the bill squeaked through on a 219-212 vote on June 26. The Senate Environment and Public Works Committee is expected to take up its version of the bill in late October or early November.

Coral Davenport covers environmental policy for Congressional Quarterly.