Democrats have certainly not been very effective in uniting behind contentious issues. Yet the Rethuglicans have strict party discipline and can force every member to sustain the filibuster on any and all issues. Why the disconnect? Why could Rethugs govern with impugnity with 51 votes while Democrats couldn't work with 60?
It boils down to the raison d'etre for each party today. The Republican party is no longer a big tent. It has a strict ideology, and it maintains power via corporate money and very good marketing. So when one of them gets elected, they're as much part of a team as a linebacker on an NFL team. The individual players don't make up their own plays; they're part of the team, and they do what the coach says.
Not every politician wants to do that. Hence we have Democrats.
The Democratic Party doesn't have ideology. Yes, we have a majority view within the caucus on many issues. But the caucus doesn't vote as a block the way the Rethugs do. Blue Dogs are welcome.
So why do the Blue Dogs join? Being part of a party is how you get power. Leadership roles go to party members. Campaign money comes from party committees. Single-seat constituencies mean that you need a plurality in your district in order to win. That creates a two-party system. And if you're not comfortable taking orders from Jim DeMint and his buddies, you're welcome to be a Democrat.
This is how we won a majority. We have a party of convenience, a place where politicians can work together to create a majority, which then allows them to divvy up the spoils of majority status. Ideology is occasionally a tool to win district elections, but not a unifying force among Democrats. Being in the the majority caucus is the unifying force.
We have to recognize this when formulating positions. Politicians who don't have a unifying ideology still have a need to make deals. We need to prioritize our deals and make the horse trades that will get members of the caucus to sometimes vote for the party position, even if it's not their own. Party discipline is the Rethug's stick approach; Democrats have to work with carrots. It is hard, and we have to recognize the difference.