WASHINGTON -- President-elect Barack Obama warned Thursday of dire and lasting consequences if Congress doesn't pump unprecedented dollars into the economy.

Video: Speech Uncut | Obama Presidency

Saying that 2009 will mark a clean break from a troubled past, Obama made an urgent pitch for his mammoth spending proposal in his first speech since his election.

"In short, a bad situation could become dramatically worse" if Washington doesn't go far enough to address the spreading crisis, Obama said as fresh economic reports showed an outlook growing increasingly grim.

Obama said that without big-time federal stimulus dollars now, the nation's recession could go on for years.

"I don't believe it's too late to change course, but it will be if we don't take dramatic action as soon as possible," Obama said in a speech delivered at George Mason University in Fairfax, Va., outside Washington.

He conceded, however, that "it will not come easy or happen overnight," and that it's likely "things could get worse before they get better."

Since his November election, Obama has deferred to President George W. Bush on foreign policy matters such as the Middle East. But, with the urgency of the economic crisis, Obama has waded deeply into domestic issues as he works to generate support for his plan to create jobs, jolt the economy and make long-term investments in other areas.

Obama laid out goals of doubling the production of alternative energy over three years, updating most federal buildings to improve energy efficiency, making medical records electronic, expanding broadband networks and updating schools and universities.

"The very fact that this crisis is largely of our own making means that it is not beyond our ability to solve," he said. "Our problems are rooted in past mistakes, not our capacity for future greatness."

The speech marked Obama's highest-profile effort yet on the flailing economy.

"A bad situation could become dramatically worse," Obama said, adding that a worst-case scenario could include double-digit unemployment and $1 trillion in lost economic activity that recalls the days of the Great Depression.

It's the fourth day in a row that Obama has made a pitch for a huge infusion of taxpayer dollars to revive the sinking economy.

Obama's stimulus package, which could cost as much as $775 billion over two years in tax cuts and spending, is expected to include tax cuts for businesses and middle-class workers, money to help cash-starved states with Medicaid programs and other operating costs, and a huge share for infrastructure building, investments in energy efficiency and a rebuilding of the information technology system for health care.

Obama's proposed tax cuts are running into opposition from senators in his own party who say they won't do much to stimulate the economy or create jobs.

Senators from both parties agreed that Congress should do something to stimulate the economy. But senators emerging from a private meeting of the Senate Finance Committee on Thursday said that tax portion of Obama's stimulus plan is unworkable.

They were especially critical of a proposed $3,000 tax credit for companies that hire or retrain workers. Democratic Sen. Kent Conrad of North Dakota described it as "misdirected."

The cost of the economic rescue package Obama wants is expected to swell to more than $800 billion or more. About $300 billion of that would be tax cuts or refunds for individuals and businesses.

On Thursday, Obama urged Congress "to work day, night and on weekends if necessary" to pass a revival plan within the next few weeks so that it can be ready for his signature shortly after he takes office on Jan. 20.