If you're running for the nomination of the Democratic party, then you *have* to represent the Democratic party, Otherwise you won't get the votes of Democrat primary voters. That is just basic reality.
Once the candidates are in the general, THEN you can talk about who best represents AMERICA. That's what the general election is supposed to be about. But right now the fight is about who represents DEMOCRATS.
BTW, why is it that it is almost always the Democrats who are supposed to stop being so mean and harsh and partisan? Insofar as Democratic politicians stop being partisan and start being above the fray, that means I as a Democrat don't get represented and my views are shut down.
But somehow nobody ever complains about how mean and harsh and partisan the REPUBLICANS are. Somehow, the Republicans get to be as mean and harsh and as partisan as they like to be. Somehow, nobody thinks it's strange that Republican voters, such as evangelicals, can go to the polls and insist that their candidates be a certain way. But when Democratic voters do it, everyone starts having fainting spells.
The fact is, there's NOTHING wrong with being partisan. I vote for a party or politician; they represent my views. You vote for your party or your side; your views get represented. Then all of these views get hashed out in the Congress and the general elections. What's wrong with that? That's what a Democratic Republic is supposed to be about.
Saying that I shouldn't be partisan is saying that I shouldn't have views. It's saying that I should shut up. That's what that's all about.
What's really going on here is a long standing effort to put down and suppress Democratic and liberal views, by condeming them as 'partisan'--(and also as demagoguery and 'class warfare' and socialism and all the rest of it). And Obama, whether he knows it or not, is at least partially going along with that effort.