What is causing this? I suspect that one reason is that more people than ever before are being tempted to participate in political arguments. The teachers in our schools, and especially our colleges, have for decades been egging their students on to engage in politics -- and, of course, providing them with a (usually radical) point of view to battle for. In addition, the growth of the Internet has provided an arena in which any political viewpoint can be aired, without restraint and without any support or justification whatever. It used to be that the average person's only means of political expression was to write a letter to the local newspaper. Now he (or she) can make any assertion whatever and blare it forth electronically, nationwide.
But that is only part of the problem. In normal political discussions, a statement (or action) that is wildly over-the-top will be sharply rebuked, and the person responsible for it quickly learns not to make that mistake again. But an anonymous blogger on the Internet or a masked protester in Portland, escapes all effective personal criticism. They enjoy all the privileges of a suicide bomber, but without the suicide.
Insofar as these problems are the result of technological innovations, there is probably nothing that can be done to stop them. But the American people had better make it clear that such tactics (and assertions) are counterproductive, or our politics will soon begin resembling half a dozen female wrestlers in a tub of mud.