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Brunner's campaign told PoliticMO that Brunner's opponents were trying to distract voters from his major endorsement — earlier Tuesday, the U.S. Chamber of Commerce backed Brunner. The group specifically mentioned Brunner's work as a successful manufacturer in making the endorsement.

Brunner faces former state Treasurer Sarah Steelman and Rep. Todd Akin in the GOP primary. The winner will face McCaskill in November.

The Hill rates this race a toss-up.