Of course we attack the Democrats just as much if not more than the Republicans. We can not attack the other party if we don't even have our party in line. That would literally be the definition of hypocrisy.
Uh, yes you can. Both parties are always going to do things that go against large portions of their base's wishes, whether it's for the other half of the country that aren't part of their party, or even for selfish/corrupt reasons. There will never be a perfect political party.
You can work to keep your party "in line" while also making the case against the other party. Spending a majority of ones energy on infighting is counter-productive to the goals of the left. I'm not saying not to protest, say, the DNC, but at least put more effort into challenging your ideological rivals.