Leading Democratic senators were on Wednesday set to unveil a major climate change bill, as Congress struggles to narrow differences on how to combat global warming.

Senators John Kerry and Barbara Boxer were putting forward their proposals after the House of Representatives adopted a similar text in June.

The Democratic bill, according to a draft circulating in US media, sets a target of reducing greenhouse gases from 2005 levels by 20 percent by 2020.

The House legislation set a 17 percent target.

Democrats have stressed that at this point, the Senate text -- a hefty 600-plus pages -- still could change.

US President Barack Obama has made addressing climate change a top legislative priority along with health care and financial reform. He has called for cutting greenhouse gases by 14 percent by 2020.

But the administration's calls to action have met stiff opposition from big business which fears that they will end up picking up the bill.

Many analysts say the law, which would usher in a cap and trade system for polluters, faces huge opposition in Congress and has little chance of being voted on before the key international climate conference in Copenhagen in December.

World leaders are keenly watching what the United States -- the world's biggest energy consumer and one of its top polluters -- can bring to the table at Copenhagen, as they seek to hammer out a new pact to replace the Kyoto Protocol which expires in 2012.

Kerry, who leads the Foreign Relations committee and Boxer, the environment committee, were due to present the bill to the media on the US Capitol steps.