The CO primary is not dependent on the base. With enough advertising money, a moderate Republican can win. McCain will not get the activist base but it is possible for him to win a primary. That depresses the vote too since many conservatives won't vote for him.
Last year, President Bush won CO while Peter Coors lost the senate election to Ken Salazar. The only way that can happen is for some GOPers to not vote or some Democrats to crossover for President but not US Senator. Since CO is more Republican than Democrat, it suggests the former happened.
Okay, that makes sense... AZ we have a primary by party and I think they still have the same thing in California, too.. every state has such different rules.