New York Sen. Hillary Rodham Clinton enjoys a 15 percentage-point lead over Illinois Sen. Barack Obama for the Democratic presidential nomination, according to a USA TODAY/Gallup Poll released Tuesday.

Among Republicans, former New York mayor Rudy Giuliani maintained a 14-point lead over Arizona Sen. John McCain.

Clinton is the choice of 38 percent of the Democrats and Democratic-leaning voters surveyed, up 7 points from a survey taken three weeks earlier.

Obama is at 23 percent, 3 points lower than the previous result.

Giuliani is supported by 34 percent of Republicans and Republican-leaning voters surveyed, compared with 20 percent for McCain.

Charles Franklin, a political scientist at the University of Wisconsin who studies polling, cautions against making too much of shifts in national surveys 18 months before the election.

Still, Clinton strategist Mark Penn attributes her boost in the poll to her performance in the first public debate among Democratic presidential hopefuls on April 26.

The survey of 1,010 adults was taken last Friday through last Sunday.