Nate Silver warns that "when an election is quite close, it does not take very much to shift the race from one candidate being a 60/40 favorite to it being about even." More generally:
One of the reasons that campaign stories have been so trivial lately is because if one of the campaigns has an especially strong line of attack on their opponent, or a great piece of opposition research, it does not make a lot of sense to drop it now when most voters are not paying attention yet. It is still extremely early for a general election campaign. If the period after Labor Day qualifies as the pennant race, and the summer of the general election year the regular season, we are still playing preseason baseball now.