I think Kennedy's 34% might be the high water mark as far as primary challenges to incumbents go. Pat Buchanan made some noise against Bush Sr. in 1992, but ended up with just a tiny portion of the nominating votes.

Trying to nominate an insurgent Democrat against Obama would be quixotic, and probably has more of a chance of backfiring (by making him take "friendly fire" and spend money on it) than it does of moving his own policies substantively to the left. He is what he is, and I don't know that a Hail Mary primary challenge will change that. Obama's going to be the Democratic nominee.

Now, maybe a progressive "wave" congressional election of true-blue supporters of pro-growth, demand-side, stimulative economic policy might give Obama breathing room to let his inner Paul Krugman out -- maybe he really does believe that stimulus works when there's idle capacity in the form of double-digit unemployment, but is scared to stake out a position so far toward the left pole of the debate. In that case, adding more lefties in congress moves the center of the debate, and lets him come down closer to Bill Clinton than to Grover Norquist. But there are a lot of "ifs" there.

I know I'll be backing progressive Democrats in 2012, and I'll probably be backing Obama as well. The difference between "making things worse very quickly" and "not making things better fast enough" is certainly worth my vote.