The United States Republican Scott Brown has been elected to replace Senator Edward Kennedy, who died last August.
It is a stunning upset in the state of Massachusetts, which has long been considered a Democratic stronghold. The Democrats have held the seat since it was won by John F Kennedy in 1952.
The Democrats have now lost their majority in the Senate, putting President Barack Obama's healthcare reform bill in jeopardy. In his victory speech, Mr Brown vowed to bring down the bill, unless significant changes are made.
The upset win also raises the possibility of large losses for Democrats in congressional and senate elections in November.