Firstly, it's not unique to Democrats. The GOP was making the same complaints about the Dems when they held all the chips of power. The real cause is the laziness of our political leaders. Most folks who go to Congress serve three terms or less. The thing is, that group of "most" represents about 65-or-so seats out of 535. All the rest are "serve" until you die politicians. Consider this...in 2006 and 2008 about 50 republican senators and representatives lost their seats. But it wasn't the strong conservatives who lost--it was the moderates. Moderate/liberal republicans lost to moderate/conservative democrats. In this cycle somewhere between 20 and 40 democrats will probably lose their seats, but it won't be the liberals. It will be the same ones who won in 2006. That most of them voted no on TARP and against healthcare reform won't matter. They will still lose. But the hardcore liberals will be perfectly safe, just like the hardcore GOPers are.
Those in "safe seats" are lazy! If the Senate leadership wanted they could pull the cots into the chamber everyday and make the opposition actually filibuster. I would love to see it. I'd love to see these chain-smoking, hard-drinking senators shaking from alcohol/nicotine/cocaine withdrawals passing out on the floor. Laugh my butt off, sure would.
Back in 1989, a couple reporters from ABC news surveyed the congress-critters at the time, asking them the number of hours they work in a day. The average response was 11 hours. Then they followed them around. The average workday for those folks was actually 6 hours. They were counting having lunch with their chums as work. When they go on corporate-sponsored week-long luxury cruises like the ones Speaker Pelosi and Minority Leader Boehner went on a couple months ago, they call it work. Like the corporate executives they joined together in bailing out, they see the labor and resources of this nation as their personal milking teat.
One day the control of our government will shift back to the GOP. Perhaps this year, perhaps not. And they will complain that the minority party won't let them pass anything. It's bull. They could hold filibusters every night, if they wanted to. But they won't Because in their world, you me, and all you see belong to them.