I was having a conversation with my roommate a couple days ago about ideology and the voting patterns of the ideological middle. In brief the ideology of a candidate (espeically for President) matters less and less the more moderate a voter is. Not understanding this is why moderate candidates often lose to more extremist ones.
The further from the ideological center a voter is the more likely she is to make her voting decision based upon the ideology (or position on an individual issue) of the candidate. Conversely the more moderate a voter is the less important the ideology (or position on issues) is to his voting decision. So if not ideology then what do these moderates base their decisions on?
The answer to that question is "strength." Particularly in a President moderates want strength. They want a President that will look strong. The President is both our head of state and head of nation. Moderates often vote based more upon how the President will serve as head of nation.
The irony is it is easier to look strong by holding a non-centralist ideological position. This is why so many moderate Democrats voted for Reagan. Reagan looked strong while Carter (1980) and Dukakis (1988) looked weak. This is also a part (there were many parts) to why Gore lost. Gore certainly wasn't less moderate than Bush, but he didn't project an image of strong leadership.
This is why Clark and Dean both have a good chance of beating Bush. Both can project an image of strong leadership. This is also why the RNC is trying pass the message about how Dean (current front runner) is angry. It isn't that they think voters will be turned off because of this "anger" it is an effort to get him to react and tone down and take weaker positions. If our nominee fall for that trap we will lose.