If by big tent you mean attracting moderates and independents, I don't think I agree.
The moderates and independents they pick up are the ones the GOP scares off. From where I sit, the DNC does a great job of building a big tent that appeals to a large number of sub-groups with wildly varying interests. Look at the groups that the DNC has managed to build into a coalition:

- Unions
- The LGBT crowd
- The Feminist movement
- Radical environmentalists
- Trial lawyers
- The heads of various old media companies
- The African-American civil rights movement
- Those in favor of amnesty for illegal immigrants

And that's just off the top of my head. Looking at the interests of those groups, there are a number of them that are clearly antagonistic to each other, but it sure doesn't stop them from supporting the party, nor does it stop the party from giving them what they want, at least some of the time.