UPDATED, 12:34 pm ET -- Nebraska Republican Sen. Mike Johanns will retire at the end of his current Senate term.

"We have decided not to seek re-election," Johanns and his wife, Stephanie, wrote to constituents Monday. "It is time to close this chapter of our lives." 

Johanns called state Republican officials, including Nebraska Gov. Dave Heineman, to inform them of his decision to retire. Heineman is likely to seek Johanns' seat, open in 2014. 

An announcement event wasn't planned, but the senator had already scheduled a series of town hall meetings across his state this week while the Senate is on break.

Johanns is one of just two Republican senators publicly backing former Nebraska Sen. Chuck Hagel's (R) nomination to be defense secretary. Other GOP senators have been aggressively critical of their former colleague, labeling him unqualified for the job. Hagel's nomination by President Obama was filibustered this past week as GOP opposition to Hagel intensified. Johanns replaced Hagel in the Senate.

Before he was senator, Johanns served as governor of Nebraska and was Secretary of Agriculture under former President George W. Bush. 

Several candidates have already told state Republican officials they're interested in running. Heineman can't run for the governorship again -- he's term-limited -- and will be ending his tenure as governor in 2014, making the timing of the race ideal. 

Also interested is Jon Bruning, the current attorney general, who lost to current Republican Sen. Deb Fischer in the Senate primary in 2012. State treasurer Don Stenberg, who's run for Senate four times before, could also get in, as could U.S. Rep. Jeff Fortenberry.

Here's Johanns' full letter to constituents:

Dear Fellow Nebraskans,
    We would like to share with you a decision we have made about running for re-election to the United States Senate. We have decided not to seek re-election.
    Words are inadequate to fully express our appreciation for the friendship and support you have given to us over the past three decades.
    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. 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.
    During these many years, we have cherished our time together. So as we think about the next stage of our lives, we want a quieter time with our focus on each other, our family and our faith. We are also confident that there will be many more opportunities to serve our state and our nation.
    We look forward to the remaining time in the Senate. It is an honor to have served in so many ways over so many years.
    May God continue to bless Nebraska and our great nation.
                        Sincerely, Mike & Stephanie Johanns 

This story was originally published on Mon Feb 18, 2013 11:35 AM EST