Greg Sargent raises an important point. Democrats act like they've been losing the broader political fight over tax cuts when, in fact, polls suggest they are winning. Every survey shows Americans oppose extending tax cuts that benefit only very wealthy people. So why aren't the Democrats forcing the issue? Because they're back to acting like 'fraidycats.
At risk of overgeneralizing, the problem isn't that Dems aren't capable of winning an argument. It's that they don't think they're capable of winning a protracted political standoff, even on an issue where the public is on their side, once Republicans start going on the attack. They seem to set their goal early on at salvaging a compromise, rather than going for the win. As a result, they tend to telegraph weakness at the outset, sending a clear message that they'll essentially give Republicans what they want as long as they can figure out a way to call it a compromise.