Quite unexpected, given he had been talking about running for re-election for the last several months. He's also a freshman running in a reliably Republican state.

Politico:

Nebraska Sen. Mike Johanns announced Monday that he would retire in January 2015, a surprise decision by a first-term Republican senator who was considered in a strong position for reelection next year.

The retirement of Johanns - a former GOP governor of Nebraska and one-time secretary of agriculture - gives Democrats a shot at a seat they ordinarily would have dismissed, but Republicans remain favored to hold the seat in a state that is reliably Republican.

Despite only serving one term, Johanns, 62, had begun to accrue seniority among an increasingly junior Senate Republican Conference, holding seats on prominent committees, including Banking and Agriculture. But after pushing for a major deficit-cutting deal in the last several years, the Republican has grown increasingly frustrated at Congress's perpetual stalemate and its inability to resolve major national problems. Johanns informed his staff and Republican Gov. David Heineman of his decision Monday morning, sources said.

The early retirement decision caught Republican leaders by surprise - given that he had signaled for months he had planned to run again. But the early announcement - the fifth senator to do so this cycle - gives both sides a chance to reassess the landscape ahead of 2014.

The GOP has a very deep bench in Nebraska and any number of candidates could almost certainly win. Current Governor David Heineman is term-limited and will step down in 2014. If he decides to run, that may limit the field somewhat.

But some House members may give it a shot regardless of what Heineman does. Rep. Jeff Fortenberry is mentioned as a possible candidate, along with Reps. Lee Terry and Adrian Smith. Another potential candidate is conservative Don Stenberg who got support from Jim DeMint during the primary of 2012. Stenberg finished third behind eventual winner Deb Fischer.