Mr. Bush now confronts the first Democratic majority in the House in 12 years and a significantly bigger Democratic caucus in the Senate that were largely elected on the promise to act as a strong check on his administration. Almost any major initiative in his final two years in office will now, like it or not, have to be bipartisan to some degree.

For six years, Mr. Bush has often governed, and almost always campaigned, with his attention focused on his conservative base. But yesterday’s voting showed the limits of those politics, as practiced — and many thought perfected — by Mr. Bush and his chief political adviser, Karl Rove.

In the bellwether states of Ohio and Pennsylvania, two Republican senators, both members of the legendary freshman class of 1994, were defeated by large margins. Across the Northeast, Republican moderates were barely surviving or, like Senator Lincoln Chafee of Rhode Island, falling to Democrats who had argued that they were simply too close to a conservative president.

Most critically, perhaps, Republicans lost the political center on the Iraq war, according to national exit polls. Voters who identified themselves as independents broke strongly for the Democrats, the exit polls showed, as did those who described themselves as moderates.

Senator Olympia J. Snowe, a Maine Republican who was re-elected yesterday, said that with the election’s results, the administration’s Iraq policy “has to change.”

“It absolutely has to change,” Ms. Snowe said. “And that message should have been conveyed by the administration much sooner.”

Mr. Bush’s allies could argue that history was working against Republicans, that in a president’s sixth year in office, his party was ripe for big losses. They could also argue that Congressional Republicans brought their own vulnerabilities and scandals to the table. But this was a nationalized election, and Mr. Bush and Iraq were at the center of it.

Nearly 4 in 10 voters said they saw their ballot as a vote against Mr. Bush, about twice as many as those who said they had cast their ballots for him. It was a remarkable turnaround for a president who just two years ago emerged triumphant from his re-election campaign, declaring that he had earned political capital and intended to spend it.

That capital slowly drained away with an ill-fated fight on Social Security, a furor over the government’s mishandling of Hurricane Katrina, an aggressive intervention for conservative causes like the right-to-die case of Terri Schiavo, and, more than anything, pollsters said, the war in Iraq. In the final days of the campaign, Mr. Bush’s travels to some of the most Republican and least competitive regions in the country were a portrait of his political isolation.

Geoffrey Garin, a Democratic pollster, said, “An important feature of this election, with implications for 2008, is that the center of the electorate clearly doesn’t like to be ignored in an era of base politics. The Republicans played to the base at their great peril among the middle.”

After a campaign that only escalated the tension between Mr. Bush and Congressional Democrats, the president will now face overwhelming pressure to take a more conciliatory approach. For example, he will be under increasing pressure to re-evaluate his support for Defense Secretary Donald H. Rumsfeld, which he so publicly restated in the closing days of the campaign.

Bruce Buchanan, a political scientist at the University of Texas at Austin, said Mr. Bush certainly had the capability to practice more bipartisan politics; he governed that way often in Texas, and also occasionally in Washington, on legislation like the No Child Left Behind Act.

Other analysts pointed out that on issues like energy and immigration, Mr. Bush can find common ground with many Democrats. Vin Weber, a former Republican congressman and lobbyist who is close to the administration, said, “They’ll be able to pivot quite easily on this and adapt to political reality.”

But much of Mr. Bush’s domestic agenda, which was not exactly gliding through the current Congress, will face even tougher prospects now. That includes any effort to overhaul entitlement programs like Social Security, already heavily shadowed by his failed effort to push through private investment accounts for Social Security in 2005, as well as any effort to extend all of his tax cuts, which Democrats say were heavily skewed to the most affluent.

Moreover, with a greater Democratic presence in the Senate, Mr. Bush will have far less latitude in his judicial nominees.

Even if Mr. Bush makes the grand gestures, Democrats heading into the 2008 presidential campaign may not be in the mood to reciprocate. Still, on Iraq, some change is almost inevitable, analysts say.

There is already a vehicle for a new bipartisanship, experts noted. A commission headed by James A. Baker III, former secretary of state, and Lee H. Hamilton, former Democratic representative from Indiana, is exploring policy alternatives for Iraq and is expected to make recommendations this winter.

House Democratic leaders have already indicated that they will not cut off financing for the war; in many ways, their greatest power will be their ability to investigate, hold hearings and provide the oversight that they asserted was so lacking in recent years.

Experts point out that Mr. Bush is hardly the first president to confront a House controlled by the opposition; since World War II, some form of divided government has been the norm. President Bill Clinton, through a combination of negotiation, brinksmanship and bluffs, produced major legislation with the Republican Congress after 1994, including an overhaul of the welfare system and a huge balanced budget law.

Mr. Bush could try to do the same. But first he would have to abandon the political worldview that he drew, by many accounts, from his father’s defeat — to never cross his base. President George H. W. Bush lost conservatives when he broke his “no new taxes pledge.”

The younger Bush has rarely made that mistake. His circle had clearly hoped that the conservative base would come through in the end, saving the Republican majority even in the face of an unpopular war. But this time, it was not enough.