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Minnesota Senate Majority Leader Amy Koch on Thursday resigned from her leadership post, shocking Republican legislators and State Capitol leaders.

"I never thought that I was doing this for a lifetime," said Koch, R-Buffalo, adding that she will serve out her Senate term but not run for re-election next year.

"It was gratifying. We did wonderful things. It was exhausting. I think you just know when it is time to let someone else have the reins, and it's time," she said.

Koch said her resignation as majority leader stemmed from her decision not to run for re-election.

"We cannot afford a lame-duck leader in negotiations next session, which is why I am resigning from my position as majority leader," Koch said in a letter addressed to "friends" and released on Thursday afternoon.

She had led the Senate for only a year. Republican legislators said they had no inkling she was planning to leave her post. Now they will scramble to elect a new leader by the end of the year, just a month before the 2012 session begins.

Koch's abrupt decision follows closely the sudden resignation of Minnesota Republican Party chair Tony Sutton. Although unrelated, both change the party's path going into the tough 2012 elections.

"Obviously, any time there are significant changes in an organization, challenges result," said Assistant Senate Majority Leader Dave Thompson, a Lakeville Republican. He said he was "blindsided" by Koch's decision.

"As long as we remain on the issues and the message and don't allow it to be about personalities, I think we'll be fine,'' he said. "We still have more votes than the Democrats do in the Senate."

Assistant Senate Majority Leader David Hann, R-Eden Prairie, said the leadership team decided on late Thursday to select Deputy Majority Leader Geoff Michel, R-Edina, interim leader until the caucus can gather to elect a new majority leader. Hann said that the election must be held within two weeks, according to the Senate's bylaws.

Hann, who ran for majority leader against Koch, said he would certainly consider being a candidate for leader but had not made a decision.

Michel did not return calls from the Star Tribune on Thursday.

Thompson, a freshman member of the Legislature, said that he had not yet thought about who might lead the Senate.

Asked whether he would think about running for majority leader, Sen. Mike Parry, a Waseca Republican who is running for Congress, said: "I don't know at this point I could answer that."

Koch, the Senate's first female majority leader, showed anyone who may follow her both the thrill and the pain of being majority leader. A U.S. Air Force veteran who worked as a Russian linguist, Koch has a quick smile during good times that disappears completely during tougher, late-night negotiations.

In 2010, she helped lead Republicans to a historic victory, leaving her safe Republican Wright County district to campaign for GOP candidates. The result: The GOP has control of the Senate for the first time since partisan elections began in 19731.

"I owe Senator Koch a lot of kudos for her helping me with my special election and her helping us into the majority," Parry said.

In 2011, Koch faced the challenge of leading a caucus filled with freshmen as well as working with the GOP-led House and DFL Gov. Mark Dayton to solve a $5 billion state budget deficit. Their failed negotiations led the state to a three-week state government shutdown that ended with a budget that pleased few.

"Any time you go through a bruising situation like that, well, people get bruised," said Sen. Sean Nienow, R-Cambridge.

He and other Republican senators, however, said that Koch survived and that they had not heard of any internal dissension or challenges over Koch's leadership post.

DFLers and Republicans alike said they respected and appreciated her leadership.

"She has been an excellent leader of her caucus and, while we often disagree, a strong advocate for her beliefs," Dayton said. He said he personally regrets her decision to step down from leadership and not to seek re-election to the Senate in 2012.

Senate Minority Leader Tom Bakk, DFL-Cook, echoed those sentiments.

"I'm actually sad to see her go," he said. "It was never, ever personal between her and I."

Rachel E. Stassen-Berger • Twitter: @rachelsb