Julian Zelizer: Three Hurdles for Obama, Romney

Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and of the new book "Governing America."

Princeton, New Jersey (CNN) -- With the party conventions over, the third chapter of the presidential contest begins. First there were the primaries, then there were the conventions, and now comes the fall campaign. During the next two months, there will be multiple hurdles for the candidates to overcome.

Given how close this race is, and the handful of states that will determine the outcome, each of these three challenges has the capacity to produce a significant swing in November:

1.) The advertising wars: The campaigns and independent organizations have amassed huge war chests. Whether voters like it or not, the truth is that much of the fall revolves around character assassination on television and radio, and now the Internet. Both parties will continue their efforts to define their opponents in the most negative light possible.

In 1988, Vice President George H.W. Bush used advertising to turn the public image of Massachusetts Gov. Michael Dukakis into that of a bleeding-heart liberal who coddled criminals and refused to be tough against the Soviet Union....