We might hope that the Democrats in Congress might stop such a thing from happening, but that isn't the case. Bush can make the decision to commit troops to war unilaterally. Then he can hold those troops hostage and blackmail congress into supporting his war, just as he has done so far in Iraq. The only power congress has against a commitment of troops is to withhold funds for their mission. But as Bush has shown in Iraq, that is politically impossible once the troops have been placed in harm's way. As soon as bullets begin to fly, all a President has to do is accuse Congress of not giving the troops the tools they need to defend themselves. The accusation of not supporting the troops is stacked in favor of the the President.
The decision to go to war with Iran would be made with the assurances by the administration to congressional Republicans that a new war would rally the country and assure them victory in 2008, which it might. Voters are easily manipulated in the short term. A large enough number congressional Republicans are short-sighted enough that they would go along with the administration and kill any timid opposition the Democrats might manage.
A third war, against a country more populous than Iraq and Afghanistan combined and far better armed, would destroy our army, unify the Middle East against us, isolate us in world affairs, collapse our economy, and end our democracy. The Republican Party would be the first to pay the price; the Democrats would not survive much longer. I firmly believe this. There is no guarantee that our form of government will survive forever. As Eastern Europe showed, when change comes, it comes quickly. The name and territory of the United States would probably survive such a catastrophe, but out real values and system of government would not. If Enlightenment democracy in the US were to die, it would also die in many other parts of the world. This war could be enough to start that domino theory.
The Democrats alone cannot stop Bush from starting another war, even if they had the will. However, if Republican leaders in congress (especially the Senate) made it clear that they would join the Democrats in resisting Bush even to the point of impeachment during a presidential campaign, the war plans would be off the table and burned. Basically, Orin Hatch and Arlen Spector have the power to prevent this war.
Should we count on the patriotism of Hatch and Spector to save us, or should we start sending pictures of puppies to Bush hoping to distract him till his term is over? My money is on the puppies.