WASHINGTON -- Senate and House negotiators yesterday turned to the thorny business of attempting to reach a compromise on a 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 that was 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. That means that 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 as well. President Barack 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 will be engaged in a high-risk balancing act. A tilt toward one group risks losing support from another.

But there is one overriding 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 is 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. Mr. Reid said the three Republicans, Maine Sens. Susan Collins and Olympia Snowe and Pennsylvania's Arlen Specter, would be working with Democrats during the negotiations "all the time."

Ms. Collins suggested that the centrist Republicans and Democrats were working together as a group, saying, "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."

Mr. Specter, interviewed later by MSNBC, said he would not back the final bill if it costs more than $780 billion -- the amount that Senate moderates agreed upon 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, was tenuous. "Everybody understands that Senator 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," Mr. Nelson said.

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

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

House Speaker Nancy Pelosi, D-Calif., yesterday said 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 sounded less confident that significant progress could be made quickly.

Monday night, a coalition called the New Democrats, which make up the largest bloc of House moderates, sent a letter to Ms. Pelosi saying there were several aspects of the Senate bill that the coalition favored over the House version, including provisions relating to broadband technology and health care. The group is led by Reps. Melissa Bean, D-Ill., and Ellen Tauscher, D-Calif.

Progressive House Democrats also pressed their case. House Education Committee Chairman George Miller, D-Calif., yesterday said $14 billion that was set aside in the House bill for school construction and modernization, but later deleted from the Senate bill, needed to be included in the final measure. He said the provision would create 315,000 jobs nationwide.

Mr. Miller may have some assistance in that effort from the White House. Mr. Obama and his economic team have talked about the importance of school modernization to the overall economic package.

There may be some room for House Democrats to add some spending provisions if the Senate were to agree to drop a $70 billion "fix" to the alternative minimum tax that is not in the House bill.