Kevin Drumm is confused

When you find that your understanding of the world is deficient - when people say and do things that are inexplicable to you - it could be a good sign that you're assumptions about the world are incorrect. Take Kevin Drumm:

I see that ABC News is running a story today about right-wing attack ads. The story acknowledges that "the nastiest rhetoric right now is coming from the political right," and Jake Tapper and Greg McCown document this with several examples. Then they end with this:

Democrats aren't necessarily running clean campaigns, though. As the races tighten in the next couple of weeks, the left will likely unleash its garbage as well.

Needless to say, they present exactly zero evidence for this.

I'm not breaking any new ground here when I say that this is, as usual, inexplicable. Sure, neither party is simon pure, but Tapper and McCown know perfectly well that the nauseating and polarized nature of modern American politics is almost entirely a Republican invention.

Which of Drumm's beliefs is wrong? Is it that "the nastiest rhetoric right now is coming from the political right"? Or is it that Jake Tapper and Greg McCown understand how journalism is supposed to work? Or is Drumm wrong in assuming that Jake Tapper and Greg McCown desire to practice journalism rather than simply acting as stenographers for whoever is willing to complain the loudest about "media bias"?