An update on primaries: Balloon-Juice picked up on this story (and links to Gary Wills doing a much better job than I do demolishing Unger's suggestion), and the comment thread is... vigorous.
One question you see in response to an expressed desire for a primary from the left (eg here) is that, since Obama is quite popular with Democratic voters (true) and would win such a primary in a landslide, what the point of such an exercise would be?
And indeed, we saw no primary challengers to Obama this cycle, despite there being any number of career Democrats (e.g. Feingold) and big-name upstarts (including Keith Olbermann) who found themselves un- or underemployed recently. The reasons not to run are not hard to find: "I don't stand a snowball's chance" is a powerful deterrent to someone whose goal is to win the election. However, I think it speaks to the skillful way that Obama has led the contingent of the party that actually holds power (as opposed to the netroots) that a primary challenge from the left, from anyone who is anyone, would be seen as an act of political disloyalty -- the first and only sin in politics.
I think this is a mistake, but it's an easy one to make, especially if Obama's like virtually everyone else who gets power in that he will use that power to stay in power.
The point of the exercise, the gain that accrues to the party and Obama's presidency from a vigorous primary, is that even if we're going to re-elect, the party should get the chance to take stock of its own successes and failures of the past term.
You don't just do a performance review when you're going to fire an employee. You do a performance review if they're still an asset to you, but need to make a few changes.
In this particular instance, the Democratic electorate needs a formal process where a challenger billing themselves as representing working-class and middle-class interests can stand up and force the president to talk about his decision to repeatedly "pivot" away from fighting unemployment to fighting the deficit. (That same person will have to give props to the president for the Lily Ledbetter act.) We need a process where someone billing themselves a defender of civil rights and civil liberties can force the president to talk about denying GWOT detainees a fair trial in open court (note: lots of Democrats in both houses of Congress ought to have a bit in their performance review on this question too). We need a process where someone billing themselves as standing up for the US being a nation ruled by laws, not men, can force the president to talk about why there have been virtually no prosecutions for a decade or more of Wall Street fraud, theft, perjury, and abuse of mathematics.
OK, that last one isn't a federal offense. But I'm considering founding a Society for its Prevention.