Democrats who want the President to be nastier to Republicans and their policies have emotion on their side, but not political logic. Obama ran for president echoing the widely-held public view that Washington was somehow corrupt and Congress was broken. He called for less partisanship and more cooperation.
Once in office, he continued striking that above-the-fray pose, with only minor deviations. Despite vigorous and often effective Republican opposition, he has achieved numerous legislative victories, though never enough to satisfy his most ardent supporters.
He can't change now. He has established his leadership persona, and he certainly has every incentive to stand apart from the partisanship and vitriol so strong in Washington today. Democrats don't like his criticizing "Congress" and not just Republicans, but that's consistent with his original campaign.
Nor is it reasonable to expect that he would be any more successful in the next 15 months, or on November 6, 2012, if he joins the mudfight now. Few of the undecideds would welcome strident partisanship from the White House.