National Journal measures the record partisan gap - caused largely by the intensification of conservative ideology:
Because about twice as many voters consider themselves conservatives as liberals, Republicans are typically less dependent on support from moderates to win elections, which further amplifies the conservative influence over the party’s elected officials. “The Democrats are always going to be fractious and divided if they want to aspire to majority status,” says Jacobson, the political scientist. “That’s just the nature of their coalition. The Republicans don’t have to be that broad; they can be much more unified. Republicans are now a conservative and very conservative coalition, and their share of moderates is minuscule. The pressure right now is not coming from their centrists; it’s coming from their extremists.”
The difference is already apparent in the early months of 2011. After last November’s landslide, House Republicans hold 61 districts that Obama carried in 2008, but those GOP members are not straying from the party agenda nearly as much as McCain-district Democrats did from their party’s priorities over the previous two years.