While bidding the Bay Area in 2002 bid versus Seattle up until very late in the process when Seattle had to drop out, people kept trying to bait me and other members of the BA2002 committee into criticizing our opposition, and vice versa, in a process called, "Let's You and Him Fight." For the most part, both of our committees avoided this particular trap, and tried to concentrate on our own positives rather than our opponent's negatives. In fact, at one Worldcon during a fairly quiet spell where the groups were getting bored, the people staffing our respective tables (facing each other across a hallway) swapped sides and for a while induced dizziness in various SMOF types (including me, when I came back from a break) by giving each others' sales pitch.
Sometimes bids have to be careful of who their friends are. We're seeing that in the US presidential election, where candidates are being embarrassed by their own supporters. I suggest that people who think that the 2011 Worldcon should not be in Seattle should concentrate on the reasons why voters should vote for another site rather than against Seattle. In my eyes, "Vote for [Site] because [Other Site] is worse" is one of the least persuasive arguments I've ever heard.
(By no means should this be construed as a criticism of renoin2011, whose leadership I know to be relentlessly positive.)