Barack Obama has opened his biggest lead so far this year over Hillary Clinton in Gallup’s daily tracking poll. Obama bests Clinton 52 percent to 42 percent in the survey conducted March 27-29. The last time one of the Democratic contenders had a double-digit lead over the other was in a Feb. 4-6 survey when Clinton was ahead by 11 points. The margin of error is 3 points. In the general election match-ups: not much change. McCain is ahead of Clinton by 4 points and Obama by 3 points, with a 2 percent margin of error.