Dean

One explanation, by Josh Marshall, of Dean's popularity (snippet):

The Republicans hold the two houses of congress by slender majorities. But they are running the place like they have two-thirds majorities in each house. They are pushing through all sorts of things and the Democrats can't really get their act together to stop any of it. With the exception of holding up some judicial nominations they are virtually impotent. And that doesn't at all reflect the reality that the country remains extremely divided along partisan lines. They're playing by an outmoded set of rules, operating on a defunct system of party discipline, and are generally getting creamed. The outrage this spawns is the wind that is filling Dean's sails. It's one big, collective: Enough! In a sense it goes beyond the Iraq vote which has gotten so much attention in this race. Almost all Dean's competitors in the race are compromised by that collective failure. Fair or not, there's a truth to it.

Yeah, but that's comparing apples (Democratic Congressmen/women) and an orange (President/candidate)