I'm not going to link directly to Peter Wehrer's article because I don't want to give him 7 more hits of traffic from my readers, but I will link to the articles written to refute his contention that the modern Democratic Party has gone too far Left.
One chart involving the US House (which is reflective of the partisan shifting in the Senate):
The Republicans have dramatically shifted more conservative than the Democrats shifting more liberal.
So in short: the Democrats have not gone too far Left. The Republicans have gone too far Asshole.