Whether it was right or wrong for Kerry to make the remark is immaterial. What is important is: 1) whether the story has legs, 2) whether a substantial proportion of likely voters believe it was a low blow, and 3) whether those voters who believe it was a low blow will be decisively influenced by the event when they cast their vote.
So far the answers seems to be: 1) yes, 2) yes (see quoted ABC poll) and 3) unknown at this time. I still tend to be skeptical about whether this will truly have any effect (as the DUI story did in 2000), but I didn't think the story would have legs either
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