I don't really understand the over-the-top contempt/hatred for John Edwards from same people who defended/have forgiven B. Clinton for same.Jonathan Bernstein suggests that there are important differences based on the concept of representation. Clinton was known to be a philanderer and basically just promised to put that aside during his presidency. Edwards, conversely, made family and loyalty to his ill wife a central part of his campaign. His betrayal of Elizabeth was a betrayal of the whole purpose of his campaign.
Let me suggest another important distinction: In 2008, Democrats had the experience of 1998 to reflect upon; in 1998, they did not.
In terms of martial problems, Clinton's nomination and election in 1992 were highly unusual. Certainly, we'd had philandering presidents and presidential candidates previously, but rarely had it been such an open issue in the middle of a contentious nomination battle, and almost never had the purported philanderer not only not dropped out of the race (a la Gary Hart 1988), but actually won the nomination. Having dealt with the issue in the winter probably, as Jon suggests, helped to inoculate Clinton for the fall campaign. And yes, there was an implicit deal that active Democrats would not make a big deal out of this issue and that Clinton, in turn, would keep his pants zipped while in the White House.
Now, when Lewinsky's name became public, yes, Democrats ultimately defended Clinton. The Republicans made that easy by determining that oral sex was an impeachable offense. (Yeah, perjury, sure.) But before the impeachment hearings started ramping up, Democrats, particularly the active Clinton loyalists, were hugely pissed at him. He had broken the contract. They'd taken a chance on him but he'd betrayed their trust. Everything they'd worked for for six years was now in jeopardy, and now they'd have to jump into the breach again, not to elect him or his successor, not to fight for health reform or economic justice, not to protect a legacy, but to save his horny ass. Of course, they did it, and Republican overreach, combined with the fact that Clinton's presidency overall ended up looking pretty successful, made it easy for Democratic activists to ultimately defend him, maybe even to forgive him... but not to forget what happened.
So then 2008 comes around and John Edwards draws the exact wrong lesson from history. He figures that if Clinton could get away with it, so could he. But Democratic activists did not want to go through that again, not if they didn't have to. So they dropped him like a hot potato.
Now, there are some other key differences, as well. Edwards' dalliances became public after he'd already dropped out of the presidential race. No one had to defend him. He was of no value to the Democratic coalition. Yes, when your president is under attack, you defend him, even if you find his behavior disgusting. But when a failed presidential candidate is under attack? Who cares?
Related to this, Clinton already had five relatively successful years as president to point to when Lewinsky's name surfaced. Activists could rationalize, "Okay, he's a dog, but he knows how to govern." With Edwards it was more like, "Okay, he's a dog."