Clinton & McCain caucus performances

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Why did Clinton and McCain perform so poorly in caucus states?
There was only one caucus each they won.
However, McCain only won Washington (and Puerto Rico) after every serious candidate had dropped out, and Clinton won Nevada, but even there she didn't receive a plurality of delegates.

Why do you think this is?

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I'm gonna guess that there's great grass roots involvement at the caucus level than in primaries, and those races tended to go more towards the candidates favored by the more ideological/anti-establishment end of the party.

I think this is absolutely correct.

Okay, I understand why Obama could mobilize his grassroot supporters.
But why didn't McCain have any? I thought there were millions of supporters who had been eager for decades to vote for him.