The simple truth is that over the past six to nine months, the Senate playing field has expanded to the point where there are now (at least) 15 races where a party switch is a real possibility — if not a probability.
Most of that expansion has benefited Republicans, who have effectively taken advantage of a national playing field tilted in their favor to take previously non-competitive races like Washington and Wisconsin and put them on the target list.
Cillizza points out that the Cook Political Report now rates “a whopping 18 races as either likely to switch, leaning switch or straight tossups” while the Rothenberg Political Report ranks 15 races with the potential to switch sides. And my hunch is that for Democrats, the political environment in November will be at least as bad as, and perhaps worse than, it is right now.
The pieces are in place for a repudiation of Obama, his party, and liberalism unlike anything we have ever seen, at least for a midterm election.