The problems faced by Democrats in Senate elections this fall just got bigger, as Sen. Evan Bayh shocked the political world by announcing he isn't running for re-election. He claims to be sick of partisanship in the Senate, though if Republicans win his seat in November, partisanship will simply get worse.

The challenge this poses for Democrats in Indiana and nationally isn't simply that a popular incumbent in a marginal and traditionally conservative state who was sitting on $13 million in campaign cash is hanging it up. It's timing: Bayh chose to take this step just four days before qualifying ends for 2010 candidates. (Two Republicans who are former members of Congress are already in the race). Since he appears to have kept his equivocation on running for re-election entirely to himself, there's no Democratic successor waiting eagerly in the wings.

Early speculation revolves around U.S. Reps. Brad Ellsworth and Baron Hill as potential Democratic candidates. But with so little time to make up their minds, nobody knows if a top-tier Democratic candidate will become available.

Expect Republican hyping of their chances (objectively still limited) of taking over the Senate this year to get amped up to a feral roar.