BOSTON -- A little-known Republican upended the balance of power in Washington by winning a U.S. Senate seat in Massachusetts, a result that imperils President Barack Obama's top legislative priorities and augurs trouble for his party in this year's elections. With 75% of the vote counted, Republican Scott Brown was leading his opponent, Massachusetts' Democratic Attorney General Martha Coakley 52.7% to 46.3%, according to the Associated Press, which declared Mr. Brown the winner. The Brown victory forces the White House and Congressional leaders into a mad scramble to decide how—or whether—to salvage their long-sought health-care overhaul. Rushing the bill after...