Voters in the north-eastern state of Massachusetts dealt President Barack Obama's Democratic Party a major blow by electing a centre-right Republican to the seat long held by leftwing icon Ted Kennedy, who died last year, dpa reported.
Television networks projected state legislator Scott Brown the special election winner with 52 per cent of the vote to 47 per cent for Democratic nominee Martha Coakley, the state's attorney general, with nearly 90 per cent of the vote counted.
The vote drops the Democrats to 59 votes in the 100-seat Senate, leaving them shy of the supermajority of 60 senators needed to pass most measures. The result jeopardizes the passage of wide-ranging Democratic reforms to the US health-insurance system, unless significant changes to pending legislation can lure at least one of 41 Republican senators.