LAS VEGAS -- President Obama threw his star power behind the Democrats' embattled Senate leader, who finds himself in a tossup campaign against an ultraconservative tea party favorite in America's most-watched Senate race.

Republicans have made Senate Majority Leader Harry Reid their top target in the Nov. 2 election. The minority party needs to gain 10 seats to wrest control of the Senate from the Democrats, and unseating the most powerful Senate Democrat would deal a major blow to Obama, who reminded supporters of Republican opposition to his agenda.

Reid is tied in the polls with relative unknown Republican Sharron Angle in a race that has attracted millions of dollars from across the nation...