The rumors may very well not be true. I had no reason before this story broke to assume Kerry was a womanizer, very much unlike Clinton, who already had rumors floating about in 1992. If he were forced to drop (unlikely even if he admits to an affair), Edwards is the clear leader among the current candidates, but we must not ignore the possibility of others entering the racein such a situation.
Speculate how you want but it's not going to happen. Kerry is going to have 2,200 delegates by the end of the day March 2, or 9th at the latest.
