As President Obama prepares to sign a $787 billion economic stimulus package today amid gales of Republican criticism of its cost, he is also facing quieter misgivings from liberal Democrats who say the bill does not go far enough -- and who are already looking ahead to future legislation that they hope will do more. Liberal Democrats recognize the package's scale and accomplishment, and they have defended it against Republican attacks. But they also wonder whether Obama could have used the opportunity of a large congressional majority and a moment of economic emergency to pass a bigger package, with a better chance of boosting the economy and with more of his priorities intact. As Obama moves on to issues such as health care and energy, liberals are debating how to ensure that the stimulus outcome does not define the outer boundaries of his agenda, so that future legislation is not limited, as the stimulus was, by the demands of centrist senators such as Susan Collins (R-Maine), Olympia J. Snowe (R-Maine) and Ben Nelson (D-Neb.). Some say Obama must aim higher next time, so that compromises produce a more satisfactory result. Some say he needs to take greater control of drafting legislation, instead of leaving it to Democratic congressional leaders, and needs to adopt a harder line with Republican legislators. And some say liberals and pro-Democratic interest groups such as labor unions must do a better job of pressuring moderate Republicans and conservative Democrats to back the president. "We can't suddenly say, 'Change has come,' and just talk to one another and add more demands. We have to be out there explaining in the most elementary ways why something like universal health care is good for America," said Theda Skocpol, a Harvard University political scientist, addressing a conference of left-leaning groups in Washington last week. She added: "It isn't going to happen in one week, and it isn't going to happen with one bill, with Olympia Snowe telling us what to do. It's going to be a long slog." Plenty of Obama supporters are celebrating the package. They note that while it includes less social spending than what passed in the House, it represents billions of dollars in spending for Democratic priorities such as health, education and renewable energy. "President Obama has been in office, what, 3 1/2 weeks? And to be able to pass a stimulus package of this size, I don't think anybody would have thought that possible six weeks ago," said Gerald McEntee, president of the American Federation of State, County and Municipal Employees. The White House touts the package, too, noting that it is larger than any element of Franklin D. Roosevelt's New Deal. It is "the most major, sweeping, comprehensive legislation as relates to economic activity ever," Chief of Staff Rahm Emanuel said last week. But as many liberals see it, the package is also notable for its omissions. They saw the bill as the launching of a new era of public investment, a sequel to the transformational presidencies of Roosevelt and Ronald Reagan. Obama swept into office with popular support, a sizable congressional advantage, and even conservative economists demanding government action. Obama supporters had envisioned big initiatives to rebuild schools, overhaul aging infrastructure and expand the safety net. The bill includes just under $50 billion for roads, bridges, transit and rail, less than many mayors and governors had hoped -- though the White House did manage to slip in $8 billion for high-speed rail. It includes $70 billion for a tax fix that will help upper-middle-class earners and have little stimulative effect -- added at the request of Sen. Charles E. Grassley (R-Iowa), who voted against the bill anyway. The final deal dropped a $16 billion school-construction fund in the House version, $11 billion to cover the unemployed in Medicaid, and billions in aid to states. And as big as it is, the final bill is smaller than what initially passed in the House and Senate, and it falls well short of filling the $2 trillion gap in demand that many economists foresee. With a third of the bill's cost devoted to tax cuts, the spending is $507 billion.