UPDATE: Cotton announces bid for US Senate seat in Ark.

DARDANELLE (AP) - First-term Republican U.S. Rep. Tom Cotton says he's challenging Democratic Sen. Mark Pryor.

Cotton's announcement sets up what's expected to be 1 of the most expensive and heated U.S. Senate races in the country next year.

He made his run official in front of supporters in his hometown of Dardanelle. He's the first Republican to announce a bid to unseat the state's two-term Democratic senator next year.

The 36-year-old Cotton is a former management consultant who served in the Army in Iraq and Afghanistan. He was elected in November to represent Arkansas' 4th Congressional District, which stretches across the western and southern parts of the state.

Pryor criticized Cotton last week, accusing him of alienating students, farmers and women since joining Congress in January.

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