He is referring to the latest book by Thomas E Mann and Norman J Ornstein, It's Even Worse Than It Looks: How the American Constitutional System Collided with the New Politics of Extremism.. Rosenberg's point is that the GOP seems to think it is OK to make extreme remarks like Allan West's slanderous claim that 78 to 81 members of the Democratic Party are card carrying Communists. Yes, he said it, and it was crazy but his colleagues are not willing to correct him. Mann and Ornstein are saying that the extremism is not balanced on both sides: The Republicans have become willing to tolerate dangerously cruel extremism: In fact, that was the title of their summary of the book in an short piece in the Washington Post: "Let's just say it: The Republicans are the problem". Finally someone is calling a spade a spade.
As Rosenberg points out the Democrats are in no way lily white, but the Republicans have outdone themselves in their silence when one of their own says cruel, unjust, and slanderous things about the other party. I keep wondering how long the many good people who have always been loyal Republicans will be willing to put up with this. The Party deserves better, but there are some extreme voices who are doing the Party no good by what they say.
Have a look at Rosenberg's interesting critique.