Hey, I wonder what sparked this inquiry? 
No worries of course. 
Anyways, I don't think being a moderate is useful unless you're in a REALLY right-wing district or a REALLY small area (like Montana, ND, or whatever). I also wouldn't read too much into Dukakis, Mondale, and etc. The Democratic Party was /very/ different back then.
I mean, in 2010, I think there were several indications. Halter always polled much better than Blanche Lincoln, and Sestak always polled much better than Specter. And that NC Democrat (can't remember his name) was doomed from the start for this reason even though he was in a very Republican district. And Tom Barrett didn't poll any better than his leftist primary rival. And Baldwin, despite being way more leftist than all of our competitors, didn't do any worse in the polls.
Of course, I don't have any solid numbers to back me up, so you can take this with a grain of salt. But I have just have seen very few examples of Democrats being punished for being too liberal. And I feel it's easier to be an electorally successful moderate Republican than a moderate Democrat. After all, the GOP nominated MITT ROMNEY and are even excited.
How in god's name is Mitt Romney a moderate in this day and age?
To answer the question, it really doesn't matter unless you're too extreme, which very few Dems are.
