There’s a tendency among voters disgusted by the discord in Washington to declare a pox on both parties, blaming the two equally for the partisan rancor and gridlock. That’s why a new book by well-respected political scientists Thomas Mann and Norman Ornstein is an important contribution to understanding US politics. In “It’s Even Worse Than It Looks: How the American Constitutional System Collided with the New Politics of Extremism,” the two conclude that the Republican Party bears most of the responsibility for today’s political dysfunction.