The House could vote today on a measure to cap U.S. greenhouse gas emissions, with Democratic leaders predicting a tight victory for a behemoth bill that has grown more complex with each compromise.

The heart of the bill, which now runs to 1,201 pages, is a plan to reduce emissions to 17 percent below 2005 levels by 2020. To do that, it would create a cap-and-trade system, in which polluters would be required to accrue buyable, sellable credits for all the greenhouse gases they produce.

But the bill also contains a system of caveats, safety valves and rule changes meant to satisfy unhappy Democrats. The result is legislation that could transform the U.S. energy industry -- and allow both Wall Street and the Iowa corn belt to build a side business in carbon.

Yesterday, Democratic leaders said they had gathered enough votes to win passage of the bill, which could be voted on today or tomorrow.

"Well, you never know until you take the vote, but we are making progress, and I'm very pleased," House Speaker Nancy Pelosi (D-Calif.) told reporters at her weekly news conference. Democratic leaders said the vote could yet be delayed to tomorrow by a backup of floor action or by worries that they did not have enough "yea" votes.

But any talk of confidence is a sign of a remarkable turnabout for Democrats.

Since this bill passed the House Energy and Commerce committee last month, they have been bogged down in an internal feud between coastal liberals, who supported a hard cap, and legislators from the Rust Belt and farm states. Those representatives were worried the bill would add a crushing new cost to electric power and gasoline.

They worked out a compromise this week, the second time that Democratic leaders have given ground on the bill.

The bill now gives some emissions credits free to rural electric cooperatives, so they can sell them and use the money to cushion consumers. And it gives the Department of Agriculture, not the Environmental Protection Agency, oversight over a key program. That would allow farmers to sell "offsets" for carbon dioxide that their crops soak up from the air or for reducing greenhouse gases from animal waste.

"The bottom-line problem is that EPA doesn't trust agriculture, and agriculture doesn't trust EPA," said Rep. Collin C. Peterson (D-Minn.), the Agriculture Committee chairman, who been a spokesman for Democrats unhappy with the bill.

Even Peterson, though, said he was not sure what the offset program would look like: "The truth is, nobody knows for sure how this is going to work."

President Obama yesterday pushed for passage in a speech from the White House Rose Garden. Though he mentioned the threat of global warming, Obama mostly emphasized non-environmental benefits, such as new jobs in clean energy and reduced reliance on foreign oil.

"The energy bill before the House will finally create a set of incentives that will spark a clean-energy transformation of our economy," Obama said. He added, "Make no mistake: This is a jobs bill."

Democrats still expect a large number of their colleagues to oppose the legislation, but with 256 members of their caucus, Pelosi and House Majority Leader Steny H. Hoyer (Md.) can spare almost 40 defections and still pass the bill. A few environmental groups have said the bill is now fatally flawed, but most still seem to support the bill, though with hopes that it might become more stringent in the Senate.

Republicans and some business interests have said the bill would add huge new costs and drive jobs to countries where emissions are still unregulated and free. The GOP byword for the bill has been "cap and tax."

Recent government reports have played down the bill's cost: An EPA study said it might cost an average family between $80 and $111 per year. The Congressional Budget Office's estimate was $175 per household.

But Keith McCoy, of the National Association of Manufacturers, said his group opposes the bill because so much is still unknown about how it would work. He said he is particularly concerned about the concessions made to win over Peterson, which introduced new provisions just days before the vote.

"This has happened very quickly. There are many new concepts, new programs, new schemes that could potentially cost a lot of money," McCoy said. "That, we're not comfortable with."

If the bill passes the House, the cap-and-trade system faces an uphill fight in the Senate, where it will need a 60-vote super-majority for passage. Majority Leader Harry M. Reid (D-Nev.) said today a bill could be on the floor sometime this fall.

The GOP's campaign arm vowed to hold Democrats accountable in the 2010 elections for the votes they cast on the measure.