Bias from the leftist media generally takes on more subtlety. Spiking, mischaracterization, buzzwords, angle, etc. Taking a frontal approach by placing TV ads would blow their tenuous plausible deniability on the issue. Of course, they could, but I doubt strongly they would take that approach.
I almost wish they would. It would give the right hard core proof that even the most skeptical could understand. It is my opinion that the left media already suffers from at best a suspicion of bias by the middle road voters. The net effect is a watering down of their ability to influence. All it needs is some blatant moves on their part to ruin what credibility they have left with all but the most jaded dems.