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Democratic Sen. Claire McCaskill said Monday that she will not run for governor of Missouri in 2016, ending rumors that have swirled around her for much of the past year.

"It was a tough decision in some ways, but in other ways it was really simple," McCaskill said on KCUR, a public radio station in Kansas City. "... I am convinced that where I can have the biggest impact is to remain in the United States Senate."

McCaskill personally gave over $500,000 to state Democratic committees and candidates in 2014, which helped stoke rumors she was thinking about jumping back into state-level politics. McCaskill ran for governor and lost in 2004 after unseating the incumbent in the Democratic primary.

But McCaskill's move won't leave Missouri Democrats without a top-tier candidate for the 2016 governor's race. Attorney General Chris Koster, a former Republican, said all the way back in 2013 that he was preparing a run for the state's top office, and Koster is seen by some as a rising star within the party.

"We have a terrific candidate that is ready to run for governor and will be a great governor, and I'm looking forward to working hard for Chris Koster," McCaskill said Monday.

That doesn't mean the attorney general is without drawbacks: Koster was the subject of a damaging New York Times profile last year of how state attorneys general interact with lobbyists. Koster reacted to the story by self-imposing new donation restrictions on his campaign committee and asking the state Legislature to take up ethics reform.

Koster's Republican past could help him attract swing voters in the gubernatorial race, and it has been a financial boon in the past: Major Missouri GOP donor Rex Sinquefield cut Koster a quarter-million dollar check in 2012 when he was running for his second term as attorney general. However, Sinquefield may have already selected his candidate for 2016. The donor contributed $900,000 to Republican former state House Speaker Catherine Hanaway in 2014, which was almost all of the money she raised for a gubernatorial run in that time.

Meanwhile, McCaskill's decision could also be a boon to Senate Democrats, who would rather have entrenched incumbents defending red-state seats than letting seats go open. The seat isn't even a sure thing for McCaskill, who might not have won her 2012 reelection race without help from Republican Todd Akin's infamous self-destruction.

McCaskill is one of three red-state Democratic senators, along with West Virginia's Joe Manchin and North Dakota's Heidi Heitkamp, who have been the subject of 2016 gubernatorial rumors early this year. Manchin said last week that he would decide whether to run for governor by late spring or summer and called his past four years in the Senate "the most unproductive years of my working life." Meanwhile, Heitkamp hasn't commented on the rumors surrounding her, though they did prompt some North Dakota Republicans to start thinking about changing the way the state fills Senate vacancies just in case.

This article is from the archive of our partner National Journal.

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