If it became clear that the only way Bush/Cheney could be re-elected was if al Qaeda made the right kind of attack. (It needs to make people feel vulnerable. It can't look like Bush/Cheney and the feds screwed-up and allowed a preventable attack. It can't look like the attackers were responding to the Iraq War.)
At what level would Cheney and the rest of the Bush national security team cooperate with al Qaeda to attack the United States? Remember, the question presumes there is no chance Bush/Cheney will be re-elected otherwise.
If it helps you envision Cheney's desperation assume scandals have broken that will result in many high level figures in the Bush administration being prosecuted for corruption and malfeasance. The Bush/Cheney team is desperate to manage these prosecutions so Cheney and Bush don't get prosecuted.