How can supposedly professional politicians be so shockingly bad at politics?

House Democrats have decided to make a great show of refusing to support the President's deal on extending the Bush tax cuts.  It is difficult to understand why any remotely competent politician would take on such a clearly untenable position.  The inevitable climb down is clearly going to be deeply humiliating. At the present time Democrats currently have two plausible alternatives:

1. Pass, however grudgingly, the President's tax deal
2. Dig their heals in and ensure that:
   1. The President is severely damaged (What use is a President who can't even rally his own party?)
   2. Democrats take the blame for everyone's taxes going (Can't avoid that when you willfully torpedo a bipartisan deal)
   3. The replacement deal to extend the Bush tax cuts will be done by the next Congress, in which Democrats will have even less leverage (Because that's what happens when you lose the house)

Since most Democrats are not actually certifiable, it is only a matter of time before they back down and take option 1. There are a fair number of people who seem to believe that, if they stick to their guns, house Democrats will somehow be able to negotiate a better deal.  This is pure fantasy.  The Democrats have precisely zero leverage in this situation.  Republicans already have a deal they like.  If the Democrats refuse to get on board they will be more than happy to blame Democrats for a tax hike, then force an even better deal next year. Since all of the above is obvious to pretty much everyone involved, the political tactics being employed by house Democrats seem thoroughly mystifying. The only explanation that makes sense is that this little piece of political theater is aimed, not at the general public, but at Democratic primary voters. Democrats who are losing their seats are taking the opportunity to position themselves as true guardians of the faith, just in case they want to run in 2012.  While those Democrats who survived the recent slaughter certainly don't want to open themselves to a potential primary challenge from the left by being seen as too willing support a deal that the folks who vote in Democratic primaries aren't too fond of. Everyone knows a climb down is coming, but precisely which Democrats are held responsible for this humiliation by the party faithful may have a big influence on a lot of Democratic careers.