Politico hinted this morning that Dean-o might be ready to take a run at The One, but his office issued a statement this afternoon that is, alas, quite Shermanesque. Feingold’s, however, is a bit more nuanced. I’m hoping he’ll reconsider — a Carter/Kennedy primary war between The One and some liberal darling would be traffic gold — but Herb Kohl will be facing reelection two years from now in Wisconsin at the ripe old age of 77 and Feingold may be looking ahead to that. If Kohl retires, he’s well positioned to succeed him: His name recognition is sky high and the 2012 landscape should be more favorable to a true-blue liberal with Obama turning out younger voters and minorities at the top of the ballot. In which case, yeah, obviously Feingold’s not going to make any noise just yet about primarying Obama; he needs to stay in the party’s good graces at least until he knows what Kohl will do re: reelection.

And yet … someone should challenge Obama from the left, no? It’s a simple strategic matter. There’s no way anyone will beat him in the primary, and the fear of weakening him for the general election is both real and well-grounded, but the reality of a Republican House means The One will face pressure over the next two years to bend to the right. And he might well give in occasionally, if only to pander to centrists ahead of 2012. Progressives need some leverage to pressure him the other way; a primary challenge would provide it. What I can’t figure out is whether a challenge is more likely if Obama weakens further over the six months or, paradoxically, if it’s more likely if he starts to rebound. The conventional answer is that the weaker he gets, the more probable a challenge is, but I’m not so sure: Unlike Carter vis-a-vis Kennedy, he can count on winning heavily among black voters against any primary opponent, which makes defeating him all but impossible. In that case, fearing a replay of 1980 in the general election, Democrats may back off and unite behind him as he starts to sink, knowing that he’s ultimately the only game in town. On the other hand, if the economy starts to improve and it looks like he’s a heavy favorite in the general, they may calculate that they can afford to beat him up a bit in a primary and try to yank him back to the left on key issues where he might otherwise bend. E.g., Gibbs is already hinting that The One’s “open to listening” to GOP proposals about extending the Bush tax cuts for wealthy taxpayers, an idea that was anathema to Hopenchange in 2008. A few policy reversals on that order and things might get interesting.

The question is, if not Dean or Feingold, who? I suppose Ralph Nader’s always available for a quixotic, futile, potentially self-defeating presidential run. Anyone else? Here’s GOP pollster Bill McInturff calculating that, yes indeed, a primary challenge is quite likely at this point.