9% / 10% Enterprisers
11% / 13% Social Conservatives
 9% / 10% Pro-Government Conservatives
11% / 13% Upbeats
 9% / 10% Disaffecteds
14% / 15% Conservative Democrats
10% / 10% Disadvantaged Democrats
17% / 19% Liberals
10% /  0% Bystanders

These numbers make it clear why to be successful, a Democratic candidate cannot appeal only to Liberals -- even though we are the largest group, there are not enough of us to carry the day by ourselves. The good news is being the biggest, we have a reasonable expectation that any candidate we support is going to have to back what is basically a liberal agenda. The bad news is that we need the voters in those other groups to win. This is a fact of life in contemporary American politics that too many progressives are blind to.

Even if we were to get all the votes of the Conservative Democrats and Disadvantages Democrats (not necessarily a foregone conclusion), that still gives us only 44% of the registered voters. That's enough for a plurality in a three-way race where a strong third party candidate syphons off some votes, but is not enough to win in a two-way race in most circumstances. We know we're not going to get the Social Conservatives, but there's enough overlap in ideas and agenda with the Disaffecteds, Upbeats, Pro-Government Conservatives and Enterprisers that we can expect to pull some of their vote if the message of the candidate is shaped with them in mind.

This is important stuff. We simply cannot say to those people "Screw you", because that's a recipe for continuing the current disaster we're in. If we had a Parliamentary system it would be different, we could count on our percentage alone being sufficient to get the largest number of votes, and we would put together the coalition that ran the government -- but that's not the way it works here (which is why it's practically impossible for a third party to gain traction).

We should keep these things in mind when we're judging candidates on the basis of how liberal they are. I don't necessarily want the most liberal candidate, I want the most liberal candidate who can win.

[Thanks to Rhea]

Update: This gives rise to a paradox: if you're examining a Presidential candidate with an eye for who to support for the Democratic nomination, and you agree with everything they say on a wide range of fundamental issues, you probably should not support that candidate, since their appeal is likely to be specific to your political typology and not broad enough to carry the election. You should look for people who are in basic agreement with you, but who stray on certain issues, or don't quite come up to your preferred solutions. The best candidate might be the one about whom you can say on most issues: "Yeah, I guess I can live with that."

That means that no matter how many times those "match the candidate to your opinions" quizzes tell me I match up best with Dennis Kucinich, I will not even consider supporting him, since he has no chance to win the election (and, in fact, is not even a serious candidate any more than Biden, Dodd or Vilsack are).