Clear and present danger
I think we've gone over this as well. Given that the antiwar people didn't believe the Iraqi defectors, and don't believe the American and British intelligence agencies, it seems that the only evidence of "clear and present danger" from terrorism that would probably be acceptable to them is a crater somewhere in America. Unless the terrorist sees fit to write a nice warning note with a return address.
The whole point of Bush's controversial foreign policy is that these decisions have to be made without perfect knowledge, since the consequences of guessing wrong would be tens of thousands of dead Americans. That's why it's "pre-emptive."