The numbers are more like 3/8 liberal, 3/8 conservative, and about 25% in the middle. The difference is significant.
In the primaries, you are trying to win the right to represent the party. So you play to the base. Now if you were to capture 100% of that 3/8 I mentioned above, you win the primaries, because the centrists' vote won't be enough to beat you.
But -- and here's where it gets tricky -- you have to capture that 3/8 AND also appeal to the centrists once you get to the general election. That's the challenge. Right now, ALL the D's are going after that 3/8 of liberals. Once they win those folks over, they will go for the other 1/8 + 1 votes.
In 2000, Bush ran far far right in South Carolina (even going so far as to speak at that bastion of segregration, Bob Jones University) and it helped sink McCain. Then he trended back toward the middle and claimed he was a "compassionate conservative" and that there was really no real difference between his policies and Gore's. It worked! He got the votes he needed from the centrists to pull off the victory.
Bush hasn't needed to drag himself back to the right for the '04 primaries, so he spares himself that grief. And he'll have the luxury of mocking the D's for going so far left during their primary season. But, the story for 2004 is still to be written.
Edited to add:
Oh, yeah, you asked some questions, didn't you?
Being a moderate is how you convince the middle to vote for you, but that's only 1/4 of the bunch, so you need another 1/4 from somewhere to win.
Nothing's wrong with compromise, except it doesn't make good TV. And to get your base energized, you have to look like you're NOT willing to compromise on the issues that are important to your base. Like abortion rights and affirmative action for the D's.
Low voter turnout happens when there's not a compelling reason to go vote. That happens if you're just disgusted with the two candidates (negative ads help there), or if you think there's no real difference between the two candidates, or if it's raining. Seriously. Sometimes it's something just as simple as bad weather that suppresses voter turnout.