"What's unrealistic is that they are trying to fund a government with today's demands on a 1950s stream of revenue," said Robert Bixby, executive director of the Concord Coalition, a research group that advocates fiscal discipline by the government.

Tax revenues soared far beyond expectations during the economic boom and stock market bubble of the late 1990s, but budget analysts say there is little likelihood of repeating that feat in this decade.

One reason is that Bush's tax cuts of 2001 and 2003 went largely to the nation's wealthiest taxpayers, the same people who accounted for the unexpected flood of tax revenue last time around. White House officials are already counting on tax revenues to surge by at least $200 billion this year, an increase of about 10 percent, and to climb more gradually after that.

But even Bush's conservative allies have warned that will not be enough to cover the continued growth in overall government spending.

Brian Riedl, budget analyst at the Heritage Foundation, a conservative group here, estimated that deficits would remain around $400 billion through 2009 if current spending trends on Iraq and major benefit programs continued.

For Bush to fulfill his promise of cutting the deficit in half by 2009, Riedl said, the president would have to cut $200 billion from domestic programs that now cost less than $500 billion a year.

"There is no way you can reach that goal by cutting only discretionary spending," Riedl said. "You have to go after entitlements as well."

About two-thirds of the $2.3 trillion federal budget now goes to entitlement programs. The Congressional Budget Office estimates that costs for Medicare will rise $55 billion in 2005, to $380 billion. Social Security outlays are expected to rise to $540 billion, from $517 billion.

But Bush has focused almost all of his budget cuts on discretionary domestic programs costing a total of $466 billion last year. Freezing spending at current levels on the vast array of programs Washington supports — thus allowing them to grow simply at the rate of inflation — would save about $10 billion next year, according to the Congressional Budget Office; a politically difficult reduction of 1 percent would save about $15 billion.

Sen. Judd Gregg, R-N.H., chairman of the Senate Budget Committee, said neither Bush's spending cuts nor his hope of strong economic growth would be enough to close the gap.

"You can't get there from here unless you look at entitlements," Gregg said last Thursday. "It's for the same reason that Willy Sutton said he robbed banks: Because that's where the money is."