WASHINGTON - President Barack Obama's drive to tackle global warming gets a boost on Wednesday, when Democrats in the Senate are expected to unveil a bill aimed at reducing greenhouse gas emissions over the next four decades.

The Senate Democrats' draft legislation, which was circulating on Capitol Hill, embraces major elements of a controversial bill that passed the House of Representatives in June.

Both bills would establish a cap and trade system for replacing dirty, polluting fossil fuels with cleaner solar, wind and other alternative energies to power factories and oil refineries and to produce electricity.

Under cap and trade, carbon dioxide emissions would drop and companies would be allowed to sell to each other the pollution permits that would control those emissions.

Unless the draft bill is changed last-minute, Senators Barbara Boxer and John Kerry have written a measure that aims to reduce smokestack emissions of carbon dioxide by 20 percent by 2020 and 83 percent by 2050 from 2005 levels.

The short-term goal is slightly more ambitious than the House's 17 percent target.

Either way, the U.S. Congress has been criticized by many countries for advancing legislation that they say would inadequately address the global warming problem, especially with a December deadline looming for an international deal on next steps.

Senate Republicans quickly denounced the Democrats' 800-page bill.

When asked by Reuters if he could support the Democrats' bill Senator John McCain said: Of course not. Never, never, never.

McCain complained that the Democratic bill merely paid lip-service to the nuclear power industry. Republicans want to encourage the building of new nuclear generating facilities with additional government aid in the climate bill. They argue it is a necessary tool in expanding clean energy.

Republicans' harsh words could preview a divisive fight over environmental legislation, similar to the bitter struggle now being waged over healthcare reform. Their opposition underscored the uncertainty over the bill's fate this year.

In coming weeks, Boxer's Environment and Public Works Committee will have to plug in key details still unresolved in their draft bill.

Those include identifying which industry sectors are to get a fixed number of free government permits to emit declining amounts of carbon dioxide in coming years, which was the subject of intense lobbying during the House debate of its bill.

If the Senate cannot manage to pass a climate bill this year, Democrats would likely take up the debate again in 2010.

(Editing by Cynthia Osterman)