Bill Clinton ran against GHW Bush when no one else would because of Bush's high ratings coming out of Desert Storm. We know who won that one.
From Medved's article:
"Yes, Democrats have had a few off-brand, odd-ball contenders who came out of nowhere to beat bigger names – including Carter in ’76, Clinton in ‘92 and, arguably, Dukakis in ’88. But even Democrats hand nominations reliably to front-runners like Stevenson, Humphrey, Mondale, Gore and, most likely, Hillary.
"In the GOP, however, front-runners don’t just usually win; they always win.
"The last time a genuine underdog grabbed the Republican nomination was 68 years ago, when a celebrity lawyer with no political experience named Wendell Wilkie managed to stampede a divided convention."
This is not quite true. The fact is, Paul Tsongas was generally liked by the NPR crowd of the Democrat-ick party and was seen as the eventual winner, with Jerry Brown being the liberal "standardbearer". Clinton was going to be an also-ran until 60 Minutes saved his bacon.