President Bush toppled the Iraqi dictatorship in 21 days. Getting his domestic agenda through the United States Congress won't be quite so easy.

Emboldened by the rapid military success, the president plans to use his new popularity to fight for his undiluted agenda of deep tax cuts, Medicare reform and strict limits on domestic spending. But the Iraq war has had an equally profound effect on the opposition. Democrats have concluded that the only way to challenge the popular war leader is to fight him vigorously on domestic policy.

The result is an approaching train wreck, officials in both branches of government say. With neither side willing to compromise, they expect plenty of fireworks and not much progress on domestic priorities this year. Though he enjoys the support of seven in 10 Americans, Bush already has about as much power as he can get in the closely divided Congress, where the loss of just two GOP senators' votes can sink his proposals.

"I don't know how he could come out of this with more power," House Majority Leader Tom DeLay (R-Tex.), a strong Bush ally on most issues, said in an interview.

The dilemma was on full display Friday. Senate Finance Committee Chairman Charles E. Grassley (R-Iowa) announced that under no circumstances would the Senate approve a tax cut this year of more than $350 billion. His promise makes it likely that Bush's proposed $726 billion tax cut will be reduced by more than half, probably ending the president's hope of eliminating the tax on stock dividends.

The culprit? Two dissenting Republican moderates, whose votes, combined with a nearly unanimous Democratic bloc, were enough to scuttle Bush's top domestic priority.

Bush hardly conceded defeat, issuing a statement Friday night signaling his continued fight for a larger tax cut. Indeed, advisers to the president say they expect Bush to campaign vigorously for his full slate of domestic proposals.

"He'll come out very strong," said Charlie Black, a GOP lobbyist who counsels the White House. As for compromise, Black said, "It's not his nature to do that." Black said that with events in Iraq calming down, Bush will likely be working the phones to lawmakers and making road trips to places such as Maine or Ohio, home of holdout Sens. Olympia J. Snowe (R-Maine) and George V. Voinovich (R-Ohio), who have so far blocked Bush's full tax cut.

White House officials believe it may not be necessary for them to win passage of many domestic policies for Bush to win reelection next year. Because the economy is stronger now than in 1991, they say, Bush is in a better position than his father was, when a successful war in the Persian Gulf was not enough to prevent a poor economy from scuttling the elder Bush's reelection.

"The state of the economy is fundamentally, totally different," a senior Bush aide said. "It's just a different reality, with fewer opportunities for Democrats to take the president down." As a result, the aide said, Bush will benefit from merely demonstrating that, unlike his father, he has a "busy agenda," even if it does not pass. That means no compromises unless there is no alternative. "The president will continue to fight for the proposals he made," the aide said.

Most economists and analysts, however, said the economy could be a hindrance for Bush if it is not growing at a rapid pace within a year.

Bush will fight, but winning is another matter. His domestic agenda is vast: the big tax cut; a Medicare overhaul with a prescription drug benefit; legislation on energy, AIDS and charities; and an ambitious plan to keep non-security federal spending to a 4 percent increase. Bush has already lost his bids to open more of Alaska's wilderness to oil drilling and to direct more government contracts to religious groups. The Senate tentatively has halved his tax cut, and his Medicare proposal faces near-insurmountable opposition. Even Bush's program to combat AIDS in Africa is stalled.

Of course, another terrorist attack or world crisis could completely change the political equation and wipe out all domestic debate. "What you hope is we don't have another incident that heightens security issues to the level that they are the only thing that matters," said House Majority Whip Roy Blunt (R-Mo.).

Barring such a turn of events, the White House faces a twofold problem: a newly unified Democratic opposition, and a few cracks in GOP unity on Capitol Hill as lawmakers decide that their own interests occasionally diverge from Bush's.

With the war wrapping up, Democrats feel liberated again to band together to wage a fight over domestic policies at home. "I believe we are only going to enhance the cohesion," said House Minority Whip Steny H. Hoyer (D-Md.). Democrats can paper over differences inside their party by opposing Bush and providing members the room to offer their alternatives, party strategists said.

Sen. Zell Miller (Ga.) is the only Democrat backing the Bush tax cut, and the president's Medicare plans have gotten a cool reception from both parties.

Democrats say they have no reason to be intimidated by Bush's support, which was up to 77 percent of Americans in a Washington Post/ABC News poll last week, from 62 percent before the war in Iraq.

"His popularity on economic proposals is rather poor," said Democratic lobbyist Larry Stein, who recently advised Senate Minority Leader Thomas A. Daschle (D-S.D.). "There has been that kind of dual focus for a long time, and I don't think it will change until the condition of the country changes." A poll last week by American Research Group found that support for Bush on the economy was 54 percent, 13 points below his overall level of support.

Competition to bash Bush will be fierce, particularly with the Democratic presidential campaign playing out in the Senate. Of the leading contenders for the nomination, four are senators -- John F. Kerry (Mass.), Bob Graham (Fla.), Joseph I. Lieberman (Conn.) and John Edwards (N.C.) -- and one is a House member, Rep. Richard A. Gephardt (Mo.). They mostly muted or toned down their criticism of Bush during the war, but they appear ready to tear into Bush's domestic agenda -- especially his economic policies -- when the public starts refocusing on matters closer to home.

Bush's budget calls for a huge increase in military spending, which squeezes out money for health, labor and education programs. Democrats plan to juxtapose cuts in popular domestic programs with the expensive rebuilding of Iraq that Bush has proposed.

The top target will be the president's campaign to provide prescription drug coverage to more seniors. Administration officials have acknowledged that they mishandled the rollout of Bush's Medicare plans earlier this year by failing to explain his position clearly to voters and GOP lawmakers alike, and it faces an uphill, if not impossible, battle.

"It becomes very, very political and very, very difficult," said Sen. John Breaux (D-La.), a centrist and key player in the Medicare debate. Bush's earlier strategy of playing hardball with Democrats from centrist states (such as Louisiana) is backfiring, Breaux suggested, citing "bad feelings" over the way the president has wielded power in the past.

At the same time, Republicans said that Bush can expect little relief from the half-dozen maverick Republicans in the Senate, where a defection of two or three can spell defeat. These lawmakers have been more concerned with the swelling budget deficits than they are about rallying behind the commander in chief on virtually every issue. They don't need the president's help to win reelection and, if anything, GOP strategists said, they get a boost back home by cultivating the image of an independent thinker.

Senate Majority Leader Bill Frist (R-Tenn.), once considered Bush's secret weapon in Congress because of their close friendship, has irritated many House Republicans by failing to rein in the president's GOP critics in the Senate. Bush and his top aides, in turn, are seen by many lawmakers as arrogant and dismissive of Congress.

Some Republicans are beginning to see their interests diverge from Bush's. When the White House declined to include money for struggling airlines in a supplemental spending bill, lawmakers decided to add it themselves. That tension is forecast to continue as Republican lawmakers defy White House budget-cutters by backing popular programs that could help their own reelections.

"For a long time, they let the president have whatever he wanted," said a GOP lobbyist close to the White House. "Now, they're doing what they think is right." On spending issues, the lobbyist said, "there might not be the kind of unanimity people would expect."

A number of Republicans urge Bush to take a more conciliatory approach to Congress. "He has to lower his expectations," said one Senate GOP aide. "It's not going to be the resounding victory at home that was achieved on the battlefield. It's attrition, not shock and awe."

Extending the military metaphor, former Republican National Committee chairman Rich Bond said that if Bush "employs the same kind of flexibility on the domestic battlefield that he's employed on the military battlefield, his hand is strengthened."

But Bush aides say they plan no changes. Upfront compromise "hasn't worked for us in the past," and Bush's confidence in his proposals is unwavering, said one senior official.

White House press secretary Ari Fleischer, asked whether Bush has accepted that he's unlikely to get the full elimination of the dividend tax, replied: "I would not conclude that, no. The president believes that there should still be, and will fight for, a 100 percent dividend exclusion."