The Bush administration has been rightly and roundly criticized for its failure to plan for the post-Saddam era. That failure produced the Iraq chaos that has trapped us all.

But historians will be equally harsh on those, including members of Congress, who want U.S. troops to leave Iraq but don't plan for what comes after. They will be guilty of the same willful blindness that got us into the current mess.

Democrats who want to wind down the war are mostly focused on troop numbers. They don't want to use the most potent congressional tool - cutting off funds to the troops - because it is so politically explosive. But they want to start a process that will produce a time line for troop withdrawal.

So they are trying to devise legislation that would restrict what U.S. troops can do. This won't help us escape the morass.

It's not just that President Bush would veto such legislation. The more essential problem is this: Unless a troop withdrawal time line is part of a bigger plan, involving regional diplomacy, it will worsen the fighting.

A time line alone would be like the starting gun at a race: With the Americans on the way out, the sectarian factions in Iraq, backed by their Iranian and Arab supporters, would gird for the ultimate power struggle.

So anyone seeking a time line for a U.S. drawdown has to think large. The Baker-Hamilton Iraq Study Group was right: A pullout should be part of a larger Mideast diplomatic scheme that involves Iraq's neighbors, including Iran and Saudi Arabia. The United States would have to broker the process, with the aim of containing Iraq's violence and helping it cool.

The Bush administration, however, dismissed the Baker-Hamilton recommendations out of hand, in part because regional diplomacy over Iraq would require talking to Iran. So legislators need to think harder about how to persuade the president to change his mind. Current circumstances in Iraq, and in the polls, give Congress unusual leverage.

Some farsighted legislators understand the need for regional diplomacy. "Leaving Iraq is necessary, but it is not a plan," Sen. Joe Biden (D., Del.) said in a recent speech. "We need a plan for what we leave behind."

Sen. Richard Lugar (R., Ind.) and Biden suggested on a recent Sunday talk show that a bipartisan group of key congressional leaders present Iraq options to Bush at Camp David. Pundits quickly shot the idea down. But I could imagine a conversation that might grab Bush's ear:

Mr. President, military force without diplomacy can't save Iraq. We don't have enough troops for a surge, and the Iraq civil war is widening.

There is only one way to change the dynamic that is consuming Iraq and threatens the whole region. We must convince Iraq's neighbors that their stark self-interest requires them to work to dampen and contain the violence.

You have the leverage, Mr. President. Iranian leaders worry privately, and Saudi Arabian leaders publicly, that a hasty U.S. troop exit will trigger a wider religious conflict in the region, with Persian Shiites and Arab Sunnis fighting over Iraq's bleeding corpse for years.

We can make use of those fears. We can warn Iraq's neighbors that our troops will remain for now - provided Iran, the Saudis, and others confer on how to limit the Iraq fighting. We can warn that, failing the cooperation of Iraq's neighbors, congressional and public pressure for a speedy U.S. exit will soar.

What could such talks achieve? Shiite Iran worries that Sunni Arab leaders want to restore Iraq's Sunni minority to power in Iraq. The Saudis, on the other hand, fear that Iran wants to dangerously expand its power in the region. They worry that Iran will manipulate Iraq's Shiite-led government and stoke troubles in Lebanon, Gaza, or even the oil regions of Saudi Arabia.

Regional talks could reassure both Sunnis and Shiites that their deepest interests would not be threatened. Under such circumstances, Tehran and Riyadh could stop meddling in Iraq and work with their co-religionists there to tamp down the violence. Then U.S. troops could begin to draw down.

Such a conference of neighbors, backed by the United States, worked in the case of Afghanistan in 2001. At those talks, Iran and the United States worked together to stabilize the situation in Kabul.

So America's problems with Iran needn't preclude a conference on Iraq. Indeed, involving Tehran in negotiations over Iraq might facilitate progress on the issue of their nuclear program.

We are telling you frankly, Mr. President: Military action alone won't save Iraq, nor can Iraqi politicians achieve reconciliation on their own. Only a change in the psychological dynamic surrounding the war can bring hope to Baghdad. Only regional diplomacy holds hope of triggering that change.

If you won't actively, and consistently, pursue diplomacy, starting now, we can no longer support your Iraq policy or your effort to send more troops. As a bipartisan group, we will make our dismay public and tell the country why we think your policy is undercutting our troops. Waning support for a U.S. troop presence will further erode.

Mr. President, help us, help Iraq, help our troops by pursuing a policy that stands a chance. Otherwise, we must oppose a policy doomed to failure. The choice is yours.

Contact columnist Trudy Rubin at 215-854-5823 or trubin@phillynews.com. Read her recent work at http://go.philly.com/trudyrubin.