Actually, who is leading nationally really is irrelevant at this point. Because the primaries take place over a period of several months, the standing in national polls will change dramatically as the first primaries take place. As some candidates surge, other drop out and other factors come into play, those number will change from week to week. So while looking at any polls is not predictive of what will happen in the future, national polls at this point are really pretty useless other than for entertainment value.
Actually, who is leading nationally really is irrelevant at this point