All of which happened last time, when General David Petraeus and Ambassador Ryan Crocker first reported to Congress in September 2007 on the Bush administration's "surge" policy. But not this time. When Petraeus and Crocker returned to Washington last week, Congress was stymied. Subdued. Cowed. And the congressmen's "frustration," reported the Washington Post, "appeared to stem from a realization that there was little they could do to affect policy in the administration's final nine months."

The Post was right. Listening to congressmen meander through various lines of questioning over the two days of hearings, one detected a certain resignation to the idea that U.S. forces will remain in Iraq in substantial numbers at least through January 2009 and possibly well into the future.

Why the shift from an emboldened antiwar movement in 2007 to a despondent one in 2008? Democrats in Congress would say the reason is structural. They don't have the votes to overcome GOP filibusters or override presidential vetoes of antiwar legislation. But they do not say why this is the case. The reason they do not have the votes is the success of the surge. If the surge had not forced Al Qaeda in Iraq into retreat, stanched the ethnosectarian conflict in Baghdad and surrounding provinces, and created the tenuous sense of security that has accompanied and encouraged ground-up reconciliation and national political progress, GOP support for the war would have collapsed. The Democrats would have had the votes to force a withdrawal. And Bush would have been unable to stop the rush to the exits.

That didn't happen. The surge's success so far has defanged the antiwar movement--for the moment.

Petraeus and Crocker have until January 20, 2009, to build on the recent gains. Their plan, endorsed by President Bush, is to withdraw the surge troops by July, then to consolidate and take stock, with any further drawdown of the remaining force of about 140,000 dependent on conditions on the ground. If John McCain is elected president, Petraeus and Crocker will probably have the backing of the new administration. But no matter who is elected in November, a large-scale American military and economic commitment to Iraq may outlast the Bush presidency. As former Bush adviser Peter Feaver noted in Commentary, Iraq may not look the same to a Democratic president in the Oval Office as it did to a candidate on "the stage of a college gymnasium filled with delirious Democratic primary voters."

In this scenario, a President Obama or a President Clinton assumes office, listens to the Joint Chiefs and the director of national intelligence, and--presto!--realizes that despite what was said on the trail, there is no easy way out of Iraq. For strategic and security reasons the American presence must continue for some time. Then the new president recalibrates plans for withdrawal, perhaps abandoning them altogether, and in a televised address from the Oval Office, coolly explains to the country that the difficult realities of a dangerous world require the commander in chief to renege on campaign promises of an orderly and timely "redeployment" from Iraq.

Fat chance. Some wishful thinkers may not believe the Democratic presidential candidates are serious. They may think that these savvy pols are playing to an antiwar primary electorate and will become more "responsible" if one of them becomes president. They misunderstand the Democrats.

For most Democrats, leaving Iraq as soon as possible is the responsible course of action. Hence Hillary Clinton's statement during the Petraeus hearings last week: "I think it could be fair to say that it might well be irresponsible to continue the policy that has not produced the results that have been promised time and time again." Would a President Clinton suddenly decide otherwise? Would she argue that U.S. security requires a large American force to remain in Iraq indefinitely? Do pigs fly?

Yes, both Obama and Clinton have been vague about how they plan to "end" the war. And Obama advisers have been in the news backtracking from promises to end the war swiftly (Samantha Power) or writing memos envisioning tens of thousands of Americans in Iraq through 2010 (Colin Kahl of the Center for a New American Security).

But these are distractions. What matters is that both Obama and Clinton promise to end the war regardless of conditions on the ground. In a March 19 speech in North Carolina, Obama said that as president he will "immediately begin" to "remove our troops from Iraq" at the rate of one to two combat brigades "each month." He then promises to "remove all of them in 16 months" while leaving "enough troops" to "guard our embassy and diplomats and a counterterrorism force to strike al Qaeda if it forms a base that the Iraqis cannot destroy."

At George Washington University in February, Clinton promised to begin removing troops "within 60 days" of taking office, also at a rate of one to two brigades a month. So far Clinton has resisted making a promise to have all U.S. troops out by a date certain. But that is simply to allow her room to maneuver; she understands that a date certain is a promise to voters she may be unable to keep. Her main promise, like Obama's, is to leave Iraq. Which is a promise either one must keep. They must keep it because (a) each believes it's the right thing to do and (b) any Democrat who is elected will face unrelenting pressure to withdraw.

This pressure will take two forms. The first is institutional. The Pentagon brass believe the Iraq war has stretched U.S. forces to the breaking point. They already think the Bush administration is drawing down our troops too slowly. So they are unlikely to resist an Obama or Clinton administration's plans to withdraw more rapidly.

Meanwhile, in 2009 Democrats will retain control of Congress, perhaps with expanded majorities in both chambers. The Democratic caucus's antiwar bonafides are well established. Senate leader Harry Reid declared the war "lost" in April 2007, and Speaker Nancy Pelosi has long held that Iraq is a "problem" to be "solved" by withdrawal. You can be sure that the Democratic Congress will try to legislate timetables or reduce funds for combat next year, especially if the new president is a Democrat. Would a President Obama or Clinton veto a bill calling for withdrawal? Would either lobby against legislation enacting political timelines for "strategic redeployment" from Iraq? Not likely.

The second form of pressure will be political. The election of a Democratic president will be interpreted as a mandate to end the war. The pro-withdrawal faction will have energy and momentum. Failure to follow through will risk a collapse in public support. This happened to the Democratic Congress elected in 2006, which promised to end the war in Iraq and saw its approval ratings sink when it failed to do so. A president requires public approval in order to enact his policy agenda. Obama or Clinton will not want to repeat Congress's folly.

So the drawdown will begin. One hopes--one prays--that by January 2009 Iraq will have reached a point where security gains can be maintained with fewer U.S. troops. But that may not be the case. It may be that security will deteriorate as Americans leave, that militias will rearm, and Al Qaeda in Iraq will regroup as the central government fumbles to quell a situation spiraling out of control. Violence may spike. Ethnosectarian conflict may flare. The Sunnis may walk out of the government. The chaos may spread. What then? Will a President Obama or Clinton halt the withdrawals--or, more likely, accelerate them? After all, Obama and Clinton have long argued that the United States should not "referee" Iraq's "civil war."

Petraeus and Crocker's achievements in Iraq notwithstanding, Obama and Clinton have promised to leave. Either one, if elected, will most likely follow through. And it won't be pretty.