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

Johanns, a soft-spoken former Nebraska governor and secretary of agriculture 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 deficit reduction accord proved fruitless.