Who's job is it to get elected? It's the candidates.
The base can and should support the candidates in order to advance their issues. But ultimately it's the candidates responsibility to get the votes necessary to win.
Sometimes that's a painful lesson for the base. Sometimes the candidate needs to go towards the center because the net gain in money, votes, and pundit support.
Ultimately it's the candidates tradeoff.
The key thing is to accurately judge how much support you lose vs how much support you gain on a given issue.
For example, McCain is never ever going to come out in favor of strong abortion rights. Although this is a tact to the center (60% of the country support strong abortion rights) the right wings base will simply react too strongly on that issue.
Every McCain adviser would strongly discourage that move.
Another example, is the FISA vote. Obama is a smart guy. And I'm sure his advisers are well paid.
They weigh the loss in money, votes, enthusiasm of the base against the money, votes, and pundit points. And it looks like the calculator said the base wasn't going to react strongly.
My guess is that someone like Shrum added it up and it looked good.
Maybe the GOP tries to label Obama as a flip-flopper for supporting a bill that only 40% of the country supports (mostly GOP base). But ultimately the huge gain in votes, money, and pundit support was worth any loss of support by the base.
In the end the candidate is responsible for their actions. And they are responsible for the advice they take. Not the base.