Actually, the more important the issue, the more likely the Dems will vote in lockstep.
Feel free to look up the voting records yourself. In most of the votes, the Dems vote pretty much as a solid block, and more Republicans are likely to vote against the majority.
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You are basically saying that both parties do the same thing: the Democrats vote to do something, and the Republicans vote against it.
What you fail to say is that on most issues, the Republicans vote to hold a filibuster in the Senate, where 60% is needed to pass any bill. They block every proposal and nothing happens.
Then they complain about the "do-nothing Congress", when they are the chief reason nothing gets done.