Sens. Hillary Rodham Clinton and Barack Obama remain locked in a tight race nationally, while Republicans have a clear front-runner for the first time: Sen. John McCain. Clinton and Obama have solidified their strengths: change voters for Obama and strength-and-experience voters for Clinton. McCain dominates the GOP field on the issues and now has a double-digit advantage as the party's best standard-bearer. General election matchups between McCain and either Democrat are toss-ups.