WASHINGTON — Senator Mike Johanns, a Nebraska Republican who is in his first term, announced on Monday that he would not seek re-election next year, the fifth lawmaker to bow out of a Senate that has become increasingly polarized and dysfunctional.

Mr. Johanns, a soft-spoken former Nebraska governor and agriculture secretary in the George W. Bush administration, appeared well positioned to be re-elected and was not on any Democratic target list. But last year, he angrily criticized conservative groups that tried to step in and influence the Senate election in his state. And his efforts as part of the “Gang of Eight” to broker a bipartisan accord on deficit reduction proved fruitless.

“With everything in life, there is a time and a season. At the end of this term, we will have been in public service over 32 years,” Mr. Johanns, 62, wrote in a letter to his constituents with his wife, Stephanie. “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.”