Bush will not be able to force the Democratic majority in Congress to give him a blank check to continue the Iraq war. The Defense Department budget has vast sums allocated to high-tech defense development and procurement programs. Absent an Iraq supplemental, these funds would have to be diverted to support troops in Iraq. Bush cannot simply let the troops run out of food and ammunition, so he will have to start shutting down other DoD programs. This will cause the CEO's of the Military Industrial Complex corporations to howl like banshees and demand a compromise to restore their funding. The Democrats have Bush trapped. All they have to do is withstand his customary cheap threats and thuggish tactics. A longer explanation of Bush's predicament follows:
Most Americans are unaware that 9/11 was used as a green light for every imaginable category of futuristic defense spending, much of it unrelated to the fighting in Afghanistan and Iraq. Super-stealthy fighters and ships have absolutely no utility against jihadi insurgents. Anti-missile systems are meaningless for dealing with Al Qaeda. These highly profitable remants of the cold war would be directly endangered by a failure to pass supplemental funding for the Iraq war.
Because Bush is the "Commander Guy," he must keep the army fighting in Iraq supplied with the materiel it requires. If he deprives these troops of resources deliberately, he will be impeached for deriliction of duty. Thus, Bush must reallocate Defense Department funds from future-oriented programs to pay for the immediate needs of the Iraq war - if the Democrats refuse to pass a supplemental.
The dollars that power the Republican Party, and the Bush White House, come from big business. Making weapons is a very big business in America. When Bush starts taking funds away from aircraft and missile programs to pay for bullets and beans in Iraq, the CEOs of the armaments companies will scream for relief. Bush will heed their cries, or the Republicans will never see another contribution from the offended companies.
Bush has no way out. If he doesn't sign a supplemental bill, he will have to offend his most important backers. Bush will fold if the Democrats don't lose their nerve.