Let’s use the golden standard for creating arguments, modus ponens. Check out the Wikipedia article if you’re not quite sure how it works.
If voting for the war indicates bad judgment, then anyone who voted for the war has bad judgment
Joe Biden voted for the war
Therefore, Biden has bad judgment
I realized I should put a little bit more into this post to clarify things a little. The point of this post is related to Obama's main tenant of his campaign, that he has great judgment, and that his judgment is proven by the fact that he would have voted against the war in Iraq. So the premise of this argument is Obama's, not mine. I just wanted to make that clear.