In a major electoral upset, the US Republicans have snatched away the Massachusetts Senate seat from the Democrats that was held by late senator Ted Kennedy for 47 years, and immediately termed the verdict a "referendum on bad policies" of the administration.
The Republican victory is a major setback to Democrats who would lose their filibuster majority of 60 in the 100 seats Senate that was essential for President Barack Obama's ambitious health care reform bill to clear the Congress.
Republican State Senator Scott Brown defeated Massachusetts Attorney General Martha Coakley in the election for the Senate seat left vacant by Kennedy's death last year.
Soon after the results were clear, Obama spoke to both candidates in the hard-fought race, the White House said.
"The President congratulated Senator Brown... The President told Brown that he looks forward to working with him on the urgent economic challenges facing Massachusetts families and struggling families across our nation," White House Press Secretary Robert Gibbs said in a statement.
The Republicans immediately termed the verdict a defeat of Obama's policies, saying it was "a referendum on bad policies, not a bad candidate".
Terming it the biggest political upset in US Senate's history, Republican Senator Lindsey Graham said: "Today's vote was a resounding rejection of an out-of-control Democratic agenda by one of the most sympathetic electorates in the country".