The House of Representatives was expected to vote on the bill, a cherished plank of Obama's domestic platform, around 3:00-4:00 pm (1900-2000 GMT) amid Republican warnings that it would send energy prices soaring and kill US jobs.

The popular president, top aides, and senior Democrats were aggressively courting wavering lawmakers in an 11th-hour battle for the 218 votes needed to ensure passage, as one top White House ally predicted victory.

"We'll have the 218 votes," Democratic House Majority Leader Steny Hoyer told AFP in a telephone interview. "We expect that we will have the number of votes to pass the bill."

Supporters of the measure, which would create a "cap-and-trade" system to curb emissions of greenhouse gases blamed for global warming, have said action is critical to success at December global climate change talks in Copenhagen.

"Today, hopefully, we'll have a celebration of American leadership taking its rightful place," Democratic House Speaker Nancy Pelosi said cautiously as she welcomed visiting German Chancellor Angela Merkel for talks at the Capitol.

The House's "American Clean Energy and Security Act" aims to reduce greenhouse gas emissions 17 percent from 2005 levels by 2020, while creating "green" jobs and boosting high-tech environmentally friendly innovation.

The 1,200-page bill, the fruit of months of tough negotiations, would create a "cap-and-trade" system limiting overall pollution from large industrial sources and then allocating and selling pollution permits.

The Democratic-crafted bill would require utilities, by 2020, to get 15 percent of their electricity from renewable resources -- solar, wind, geothermal, and biomass -- and show annual energy savings of five percent from efficiency measures.

The European Union plan calls for getting 20 percent of all electricity from renewable resources by 2020.

White House sources cautiously declined to speculate on the vote count, or to say whether they believed the legislation would pass, while Republican sources shied away from discussing their own vote count.

But Obama's critics charged in heated debate that the legislation would impose a massive tax on energy use and kill jobs in the midst of an already painful recession.

The bill "is the equivalent to a light switch tax -- if this bill becomes law Americans will pay higher taxes every time we turn on our lights," warned Republican Representative John Culberson of Texas.

Other foes said the United States was wrong to act before securing firm commitments from rising economic powers and major polluters like India and China.

And the measure, backed strongly by former vice president turned climate change campaigner Al Gore, has also drawn fire from some environmental groups, with Greenpeace USA urging its defeat in a statement released late Thursday.

Wary of defeat, Obama, White House chief of staff Rahm Emanuel, and other advisors were reaching out to lawmakers by telephone in a bid to corral the votes needed for passage, a White House source told AFP.

"The president feels very strong about this bill. It's one of his major priorities and he is also talking to members and urging them to support this legislation," Hoyer said.

"We worked late yesterday and we're still working this morning. This is a very big piece of legislation and complex. Members are giving (it) a lot of thought."

The Senate will likely not act before mid-September, when leaders of key committees are due to make their proposals public, launching what could be a months-long process to get the measure through the Senate, reconcile any differences with the House version and pass a final bill.

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