Rep. Tom Cotton (R) dethroned Sen. Mark PryorMark Lunsford PryorTom Cotton's only Democratic rival quits race in Arkansas Medicaid rollback looms for GOP senators in 2020 Cotton pitches anti-Democrat message to SC delegation MORE (D) in Arkansas on Tuesday, putting the GOP one step closer to capturing the Senate.

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Cotton, a 37-year-old just elected to the House in 2012, capitalized on voter discontent with President Obama, who is deeply unpopular in the state. He's set to become the Senate's youngest member.

Pryor, whose father was also a senator, hoped his family name and centrist profile could help carry him through in a state that's trended hard toward the GOP in recent years.

Cotton relentlessly tied Pryor to Obama throughout the race, hammering him for voting for ObamaCare, tying him to the White House's foreign policy and accusing him of backing "amnesty" for illegal immigrants.

Pryor fought back with attacks on Cotton's views on Medicare and Social Security and his vote agains the farm bill. But the state's red trend and Cotton's aggressive campaign was too much for him.

— This post was updated at 8:40 p.m.