WASHINGTON — Senate and House negotiators yesterday turned to the thorny business of attempting to reach a compromise final version of a massive economic stimulus package — with the goal of delivering a bill to the White House by the end of the week.

It will be anything but easy. There are significant differences in the bill the Senate passed yesterday and the one approved by the House last month, not the least of which is the price tag. The Senate bill, at $838 billion, is larger than the $819 billion measure the House approved.

Beyond that, there is a clash in approaches. The Senate bill places more emphasis on tax cuts as a means of jump-starting the economy; the House bill relies on more government spending in an effort to create more jobs.

Once a final deal is struck, the bill has to pass both houses of Congress again. So the legislation must satisfy a variety of independent voting blocs: the moderate Republicans in the Senate who broke with the party to ensure the bill's passage, the conservative Democrats in the House who may favor elements of the Senate bill, and more liberal members of the House who don't want to see the bill's commitments to funding for state budgets and education be sacrificed.

And, of course, the White House will be weighing in. President Obama journeyed to foreclosure-ravaged southwestern Florida yesterday for a campaign-style economic "town hall," pressuring Congress to act quickly to produce a final bill.

"We've had a good debate, but the time for talking is over," the president said. "Folks here in Fort Myers and across America need help, and the time for action is now. The Americans I've met understand that even with this plan, our recovery will likely be measured in years, not weeks or months — but what they don't have patience for is more waiting on folks in Washington to get this done."

The congressional negotiators working on the stimulus deal will be engaged in a high-risk balancing act. A tilt toward one group risks losing support from another. But there is one trump card to be played as talks go forward: The three members of the GOP who were crucial in pushing the bill through the Senate — as well as some moderate Democrats — say they are prepared to walk away from the package if it's materially altered this week from the Senate version, or if it grows in spending.

Their threats were serious enough for Senate Majority Leader Harry Reid, D-Nev., to meet with a group of centrists in his office yesterday after the Senate passed its bill on a 61-37 vote.

Reid said the three Republicans, Sens. Susan Collins and Olympia Snowe of Maine and Arlen Specter of Pennsylvania, will be working with Democrats during the negotiations "all the time."

Collins suggested that the centrist Republicans and Democrats were working together as a group, saying that "there are a number of the more moderate Democrats who are also very concerned about the possibility of the House trying to reinsert a lot of excessive spending or the package itself becoming large."

Specter said he would not back the final bill if it costs more than $780 billion, the amount Senate moderates agreed on late last week before some additional amendments swelled the bill back over $800 billion.

And a Democrat, Sen. Ben Nelson of Nebraska, who was instrumental in brokering the compromise on the bill that led to the GOP defection, warned that his support, too, is tenuous. "Everybody understands that Sen. Collins says if this comes back materially altered as to the top-line number or to the pieces within the package, that she plans to vote against it, and you can put me in that category, too," Nelson said.

BAILOUT STRATEGY

Separately, Treasury Secretary Timothy Geithner yesterday outlined plans for spending much of the $350 billion in financial bailout money recently cleared by Congress, and the Federal Reserve announced it will commit up to $1 trillion to make loans more widely available to consumers.

The Treasury will guarantee a portion of the Fed investment by putting up $100 billion, an increase from the $20 billion commitment the Bush administration had announced.

The goal of this program is to make it easier for consumers to buy cars or obtain student loans, small-business loans or other types of credit that have dried up in recent months.

Taken together, the moves by the Senate, Fed and Treasury yesterday marked at least a political watershed if not an economic turning point — the day the 3-week-old administration and its congressional allies assumed full control of the struggle against the worst economic crisis since the Great Depression.

MORE SKIRMISHES AHEAD

When a deal is struck by Congress on the stimulus package, the final bill must be ratified in both chambers. The Senate will need 60 votes to avoid a filibuster, which gives Collins and the other centrists maximum leverage.

Reid said he hoped negotiators could have a "first cut" of the bill in 24 hours.

House Speaker Nancy Pelosi, D-Calif., said yesterday that the House bill does more in terms of job creation. "Our bill produces more jobs," she said. "We will go to conference to fight for those jobs." She seemed less confident that significant progress could be made quickly.