Take a look at this NBC First Read item on Hillary Clinton. The First Read team writes:
Lots of folks keep bringing up Howard Dean when talking about Clinton's vulnerability vs. inevitability. The comparison, though, doesn't work since Dean never led the national polls by this much - a new USA/Today Gallup poll has her at 50 percent, with Obama at 21 percent - nor had a stranglehold on the establishment the way Clinton does. The better comparison (if there is one historically and there's a chance there isn't one) is George W. Bush in 1999 or Walter Mondale in 1983. Both Bush and Mondale had HUGE advantages in the polls and among the establishment class of both parties, and then an early state loss put them in a precarious position.
You can read the USA Today / Gallup poll they mention here. Bill Kristol made a similar point about Hillary Clinton in this Time magazine column back in early January. Here's Kristol:
The examples of Muskie and Kerry are Clinton's Scylla and Charybdis. She will spend the next year trying to navigate between the twin dangers of being too moderate on the war for an antiwar primary electorate and going so far in mollifying that electorate as to weaken her chances in the general election. Like Muskie, a Humphrey backer in 1968, and Kerry, an Iraq-war authorizer in 2002, she's saddled with the original sin of being an original war supporter. Like Muskie, she's been moving gradually away from that position. Like Kerry, she'll soon have to cast votes on various legislative proposals related to the war.
Muskie, the establishment centrist, lost to McGovern in 1972. Kerry, the establishment liberal, beat back antiwar insurgent Howard Dean in 2004. We won't know until January whether Clinton is Muskie or Kerry. Of course, the way things have been going, increasingly it looks as though she can't be stopped from winning the Democratic nomination.