By kicking Kucinich, you send the message to potential young progressives that you will be weak-kneed and back off from the fight and will accept right-wing attacks on your allies.  Dennis Kucinich is but one politician that you centrist-minded so-called progressives denounce.  You denounce all lefties as not living up to your perfect standards (and a standard you don't even apply to conservatives let alone other Dems).  It's gotten so bad that your thinking has created an environment where the largest anti-war activist group in the country, one of the biggest progressive voices, was censured on the Senate floor last year.  And you accept it.   And have the audacity to blame Kucinich and true progressives for your failure.  

It's why we had a perfect storm for Democrats and liberals this year but somehow ended up worse off for it because our conservative Democratic politicians suck.  And you want to suck up to them instead of replacing them with real progressives like Kucinich.   This defeatism you display is why John Kerry, a war hero, got swiftboated by a war avoider.  It's why you will probably allow Wes Clark to get swiftboated.  He's a great attacker against McCain's strength and we'll see if Obama has the wisdom to even defend Wes Clark here, let alone Kucinich.  My bet is Obama will do what you're doing to Kucinich--attack an ally to make yourself seem more adequate and sensible.  It's a short-sighted and bad strategy even progressives are playing here.  You guys are doing it yet again.  You feel bad so you lash out at progressives.

In short.  It's not the candidates.  It's you.  You do not demand that candidates stand up for liberal positions and you shoot anyone to the left of you because they don't perfectly represent liberalism as you imagined they should.