(1) In the very unlikely event Edwards had been elected president in 2004, he would probably never have met Rielle Hunter.
(2) In 2008, the affair was too well-known to be covered up, and therefore Edwards would not win the nomination. If need be, if it appeared he was on the way to winning the nomination, some campaign staffers would have revealed it: "However, in August 2006, several staffers—including Josh Brumberger, Kim Rubey, and David Ginsberg—had strong suspicions about the affair and left the campaign over concerns about Hunter. Rubey and Ginsberg later discussed their obligation to the party to come forward with what they knew, after finding Young's paternity claim unconvincing.."
https://en.wikipedia.org/wiki/John_Edwards_extramarital_affair The mainstream media's reluctance to cover the affair was probably motivated by a desire not to make things worse for Elizabeth Edwards unnecessaeily (i.e., as long as her husband didn't look like he would be nominated).