According to local media, Hyde-Smith gained 54.4 percent of the vote, while Espy got 45.6 percent with 95 percent of ballots having been counted.
US President Donald Trump has already congratulated Hyde-Smith on the victory.
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The Senate special election in Mississippi was held on 27 November and resulted in the boost of the Republican majority in the Senate up to 53-47 seats.
On November 6, the United States held its midterm elections, after which the Republican Party retained its majority in the Senate but lost its predominant position in the House of Representatives.