Agriculture Secretary Tom VilsackThomas J. VilsackUSDA: Farm-to-school programs help schools serve healthier meals OVERNIGHT MONEY: House poised to pass debt-ceiling bill MORE will not run for retiring Sen. Tom HarkinTom HarkinThe Hill's 12:30 Report Distance education: Tumultuous today and yesterday Grassley challenger no stranger to defying odds MORE's (D-Iowa) seat in 2014, easing one potential obstacle in Rep. Bruce BraleyBruce BraleyOPINION | Tax reform, not Trump-McConnell feuds, will make 2018 a win for GOP Ten years later, House Dems reunite and look forward Trump: Ernst wanted 'more seasoning' before entertaining VP offer MORE's (D) pursuit of the nomination.

“He will not seek the open seat" Vilsack aide Matt Paul told The Des Moines Register.

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A recent poll showed Vilsack with the highest level of support of all potential contenders tested, with 56 percent of those surveyed saying they saw him as an appealing candidate.

But Paul said Vilsack will focus on his job as Agriculture secretary, instead of making a run for Senate.

Braley is currently the only announced contender for the seat on the Democratic side, and Vilsack's decision not to run makes him the front-runner. 

Republicans will see a fierce primary fight, likely between Reps. Steve King and Tom Latham, though neither has officially launched a campaign.