The unpopularity of the stimulus package and Obamacare resulted in the defeat or retirement of most of the Blue Dogs. Their numbers fell from 54 in 2009 to 26 after 2010 to only 14 in 2012. A historic Democratic constituency largely disappeared, and so did Democrats' majority in the House.
Democratic numbers were further reduced by Pelosi's decision to pass cap-and-trade environmental legislation in June 2009. That decision favored the Democrats' urban green constituency over its historic constituencies in coal and oil country.
It was particularly surprising, since cap-and-trade's prospects in the Senate were never good. So coal and oil country Democrats were sacrificed for nothing. West Virginia, once safely Democratic, voted 62 percent for the not-culturally-Appalachian Mitt Romney.
This year Senate Democratic leader Harry Reid made a similar choice when he threatened to eliminate the requirement of 60 votes to overcome a filibuster unless Republicans agreed to allow confirmation of members of the National Labor Relations Board.
This favored the party's labor union constituency, which feared the Supreme Court would affirm an appeals court decision declaring Obama's recess appointments to the NLRB invalid.
The unions want favorable NLRB rulings over the next three years. Reid acted at their behest even though there's a good chance Republicans will regain a Senate majority in 2014, in which case changing the filibuster rule would hurt Democrats.
But he wasn't willing to change the filibuster rule on judgeship nominations -- something the party's feminist constituency would love. Reid favored the unions and shoved the feminists under the bus.
Campaigning is about assembling majority coalitions. But to govern, as John Kennedy said, is to choose. In particular, governing requires choosing to favor one constituency over another. That can result in the disassembling of a majority coalition.
Democrats aren't necessarily doomed in 2014 or 2016. But they are weaker because of the choices their leaders have made.