At Politico, Douglas Sosnik writes:
The progressive domination of the Democratic Party is already being felt across the country at the state and federal levels. Much less ticket-splitting is taking place these days, as the overall political leanings of most states increasingly determine how people vote from the top of the ticket down to the local level. As a result, a growing percentage of Democratic federal and statehouse officeholders come from blue states and districts across the country where progressive views tend to predominate. Democrats elected in more purple areas often tend to be more moderate in their views, but when these seats open up due to retirements or defeats, the more progressive forces will likely determine the primary election outcomes.
Just look at the U.S. House of Representatives. Democratic House members are overwhelmingly liberal, due in large part to 2010 midterm election losses and the 2012 reapportionment and redistricting process, when they were packed into as few seats as possible by Republicans intent on keeping control of the House. As a result, moderate Democrats are somewhat of an anomaly in the House—there are currently only nine Democratic members of the House in seats carried by Romney in 2012. And this number is certain to drop due to retirements and defeats in the upcoming midterm elections. The House Blue Dog Caucus, a moderate-to-conservative group of Democratic lawmakers, currently has only 19 members—down from 54 only four years ago. This number will also decrease after November.
The last bastion of moderate elected Democrats is the U.S. Senate—but even that base appears threatened. Although there are currently 12 Democratic seats from states that Romney carried in 2012, seven of them are up for re-election in November. As in the House, this bloc will almost certainly shrink.