Clinton slipped in by running when everyone else was too scared to run. He taught every politician to never let that happen again. Now, regardless of how popular an incumbent is, there will be top names to run. And 2008 is an open seat-- no one slips by in an open seat environment. It will be a free-for-all, but Hillary will scare them away by raising $200M for the primary-- more than any GOP candidate will raise, too.
Clinton slipped in because GHW Bush violated his "Read My Lips" pledge and that Little Weasel Ross Perot. The 2nd time was a decision by the Pubbies to punt, and let Bob Dole have his last hurrah.