Let's be clear about one thing: Richard Cohen is not part of "the Left". There is a Left in our country, and it does not occupy itself with trash talking conservatives, it is busy dissenting from the consensus that both Cohen and Prager are part of.
Currently, the left is dissenting from Obama's pick of John Brennan for CIA. His torture-defending past is considered controversial on the Left, but that was not an issue with Obama nor Bush before him (Brennan served in the Bush admin), and both Cohen and Prager have vigorously defended torture in their commentary.
Cohen might trash talk the "other side" in his column but he probably had cocktails with Krauthammer that same evening.