WASHINGTON -- In a surprise announcement that adds to a growing list of Senate retirements, Nebraska Republican Mike Johanns said Monday that he will not seek a second term in 2014.

Johanns, who was elected to two terms as the state’s governor and served in George W. Bush’s Cabinet as agriculture secretary, said in a letter to his constituents that it was simply the right time to end his three decades-long career in public office.

“Between the two of us, we have been on the ballot for primary and general elections 16 times and we have served in eight offices. It is time to close this chapter of our lives,” Johanns wrote along with his wife, Stephanie, who was a county commissioner and state senator.

Johanns, who in 2008 won the Senate seat once held by Chuck Hagel, is one of just two Republicans who has said he would vote to confirm him as defense secretary. He was one of four senators who voted last Thursday to end a Republican filibuster of his nomination, which failed by a single vote.

Johanns becomes the fifth sitting senator to opt against seeking another term, joining Democrats John D. Rockefeller IV (W.Va.), Tom Harkin (Iowa) and Frank R. Lautenberg (N.J.), and Republican Saxby Chambliss (Ga.).

Johanns’ decision to retire was arguably the biggest surprise. He was not expected to face a competitive reelection bid in 2014, either in a primary or general election.

Republicans would be favored to hold his seat in the deeply conservative state. Gov. Dave Heineman, who succeeded Johanns in the office after he resigned to become agriculture secretary and is term limited, would be a leading candidate in the race.

Thirty-five Senate seats will be contested in the 2014 midterm elections. Democrats currently hold 21 and Republicans 14.

michael.memoli@latimes.com

@mikememoli