Oh really? I hear this claim a lot, but its been my observation that if the Democrats do something stupid, Jon will certainly take the opportunity to nail them to the wall. I recall that when the Democrats got control of the Senate, that he played video of them discussing an effing sports game rather than fixing the country, like they claimed they were going to. Or showing clips of similarities between Bush and Obama speeches. Or comparing Obama's exit strategy with Bush's. Or calling out Democrats on hyperbole.
Jon does clearly lean left, but I don't see him as playing softball with the Democrats. They just tend to give him less ammunition. Yes, the Democrats do have their fuck-ups, but the Republicans have taken the whole dishonest-fuck-up game to a whole new level.