Braley had long been close with Harkin, and both local and national Democrats have viewed him as the strongest and most likely candidate to replace Harkin, in the event of a retirement. Braley first won an open seat in 2006, when GOP Rep. Jim Nussle made an unsuccessful bid for governor. Braley survived a serious scare in 2010, escaping with a two-point victory in the Republican tidal wave that year, but all the while he continued to move up the leadership ranks among House Democrats, eventually becoming a vice-chair of the DCCC.
More recently, Braley held out the possibility he might run for governor next year, but the Senate always seemed like a better fit, and Democrats have to be excited to be getting an early start on holding this seat. Republicans, meanwhile, could be on track for an extremely bruising primary between wingnut standard-bearer Steve King and establishment choice Tom Latham, both, like Braley, members of the House.
No Republicans have yet declared, though a recent PPP poll had Braley leading both King (49-38) and Latham (44-41). Undoubtedly, Braley would prefer to face the much more incendiary King, but he starts off in a good position even if he faces Latham, who is the GOP's top choice.
Braley's decision will also set off a great deal of interest in his House seat, a blue-leaning district that went 56-43 for Barack Obama last year. That's probably out of reach for Republicans, but the NRCC probably won't want to give Dems a free pass, though that may be easier said than done. As always, we'll be covering future developments in both contests closely. For now, as far as the Senate battle is concerned, Team Blue is off to a good start.