"Mr. President, declaring war, or providing the authority to wage war, is the single most important responsibility given to Congress under the Constitution. As history has shown, wars inevitably have unforeseen, terrible consequences, especially for innocent civilians.
Blank-check resolutions, such as the one the President proposes, can likewise be misinterpreted or used in ways that we do not intend or expect. It has happened before, in ways that many people, including Members of Congress, came to regret. That is why a thorough debate is so necessary. And that is also why this Vermonter will not vote for a blank check for this President or any President. My conscience and the Constitution do not allow that."
this is just a snippet. read the whole speech in which Leahy compares this resolution to the Gulf of Tonkin Resolution, and accurately forecasts the future of the Iraq debacle, here:
http://leahy.senate.gov/press/200209/092602a.html