In an epic upset in liberal Massachusetts, Republican Scott Brown rode a wave of voter anger to defeat Democrat Martha Coakley in a U.S. Senate election Tuesday that left President Barack Obama's health care overhaul in doubt and marred the end of his first year in office.

The loss by the once-favored Coakley for the seat that the late Sen. Edward M. Kennedy held for nearly half a century signaled big political problems for the president's party this fall when House, Senate and gubernatorial candidates are on the ballot nationwide.

More immediately, Brown will become the 41st Republican in the 100-member Senate, which could allow the GOP to block the president's health care legislation and the rest of Obama's agenda. Democrats needed Coakley to win for a 60th vote to thwart Republican filibusters.