Senator Evan Bayh, the moderate Indiana Democrat who nearly was Barack Obama’s Vice Presidential pick (he was in the final three with Biden and Tim Kaine), has announced he will not stand for re-election this year. This is significant — he is the third Democratic Senator to announce his retirement, and in doing so feeds uncertainty in the Democratic ranks about their ability to hold onto the Senate come the midterms.

More than that, this is odd. It was entirely unexpected. Bayh was thought to be a rising star. At the age of 54, he’s already served approaching two terms in the Senate in addition to two terms as Governor of Indiana (a post he was elected to at the age of just 33); as a moderate Democrat, the political weather is very much with him, and will surely continue to be. He is often talked of as a future presidential contender. And he would have been re-elected: even after his predecessor, Republican Dan Coats, announced his intention to run against him this year, Indiana Democrats released a poll last week showing a 20-point lead for Bayh. He had a $13m war chest, which means that lead would’ve grown.

Against that backdrop, Bayh’s decision merits examination. In his announcement on Monday, he gave a simple explanation: “I do not love Congress”; and, to paraphrase, I can do more good outside of the Senate than inside it. I don’t doubt that on some level this is a genuine motivation for his decision. But it only tells part of the tale. Bayh reasons that the Senate is an institution mired in partisan bickering and inaction. It is, but it has been for years. It arguably was even when his father, Birch Bayh, served in it from 1963 to 1981. Bayh surely hasn’t just noticed this. The present reality of Democratic Senatorial impotence even with a large majority will certainly have impressed the comparative pointlessness of the Senate upon him with new vigour, but it’s hard to argue that the institution he announced his desire to leave this week is radically different from that he worked so hard to join in 1999. And more notably, Bayh is an astute operative. He wouldn’t leave if he had no plan for where he was going. This move is about a real and deep personal frustration, but there are undoubtedly other motives at play.

What might they be? The chatter about a possible run for President in 2012 is all nonsense. Bayh will not run against Obama. Aside from the fact a Democrat trying to take down Obama from the moderate wing of the party could not beat him — it would be political suicide — Bayh is too strongly tied to Obama personally and politically to credibly run an insurgent campaign.

Bayh might well be running for President in 2016 — that is far more plausible — but resigning from the Senate doesn’t obviously make sense if that were his sole intention. The Senate is not a great base for a Presidential run — your past voting record can hurt you, and the battered reputation of Congress and the difficulty in making yourself stand out from the crowd don’t help with the electorate — but there are worse platforms, and being out of office probably is one of them. In this era of the pundit candidate — the likes of Palin, Huckabee and Gingrich all spend hefty amounts of their time spouting their message on cable news in the hope of running successfully in 2012 — Bayh is not an obvious beneficiary: he’s unlikely to get a big gig on MSNBC; he’s not a big enough name or an interesting enough performer.

So he’d have to do something else in the meantime. He might do something in the nonprofit sector, as he hinted on Monday — John Edwards nearly made that work in his post-Senate career (before he became intractably mired in scandal). He absolutely should not take a lobbying job or work on Wall Street, as so many temporarily-retired pols do (see Harold Ford); it would hurt him in any future campaign. If he’s lucky, Obama might give him a Cabinet seat, but it’s unclear who would be shuffled out.

Whatever his next step is, the way Bayh’s played this latest move is surprisingly machiavellian. By announcing his retirement on Monday, he gave potential replacements in the Democratic primary less than 24 hours to file candidacy declarations (which required 500 signatures) before the deadline. No one managed it, which means the bosses of the Indiana Democratic party — who are intensely loyal to Bayh and will follow his lead — get to pick the candidate. This effectively means Bayh will get first refusal on who his possible successor will be. This is undemocratic, but a pretty astute piece of personal politics.