Editor's note: 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.
In 1996, President Bill Clinton turned the perception of Sen. Robert Dole into a Republican extremist who was only a few steps away from the most radical Republican freshmen. In 2004, the George W. Bush campaign and independent groups transformed Sen. John Kerry, in the minds of many voters, into a flip-flopping anti-war protester who had no political principles and would endanger the country in the war against terrorism.
2.) The presidential debates: These debates, which begin October 3, are likely to matter more than most. Whenever there is a campaign this tight, any push, in either direction, can make all the difference in the world. The debates will offer an opportunity for each candidate to demonstrate his competence and mastery of the issues, albeit in short sound bites. It will also be a high-visibility moment when both candidates can shine or stumble as a result of their physical actions and rhetorical skills.
There have been many moments in recent history when debates had an impact, whether it was Richard Nixon sweating in front of the television cameras in 1960 or Ronald Reagan turning to Jimmy Carter in 1980 to say, "There you go again."
When George H.W. Bush looked at his watch during a town hall meeting about the economy in 1992, it sent a signal to voters that Bush didn't actually care about the recession. When Al Gore sighed repeatedly in 2000, it conveyed an image of arrogance that played into George W. Bush's hands.
President Obama at first was not the best debater, as a result of his tendency to take a lawyerly approach to questions, outlining the positives and negatives of every position. But he put in some excellent performances against Hillary Clinton and John McCain in 2008 that showed he could use the debate stage to weaken his opponents.
During the Republican primaries this year, Mitt Romney also demonstrated that he could remain extremely poised and in command of policy debates, even if he occasionally had a slip of the tongue (like making a $10,000 bet).
3.) The "ground game" will be crucial, especially as the candidates get close to the finish line. Even in an age of the Internet and smartphones, having dedicated people working to get out the vote in communities throughout the country can have a huge impact. The candidates will need supporters to generate excitement within the electorate, to spread campaign material and get out the message, and to make sure that voters come out on Election Day.
According to most reports, Obama still has the advantage on this front. While Republicans have demonstrated their prowess with fund-raising, Democrats have done a good job of recreating some of the organizational muscle that carried Obama to a win in 2008.
One Washington Post-ABC News poll recently found that Democrats had contacted 20% of registered voters, compared with only 13% by the GOP. Republican National Committee Chairman Haley Barbour said, "That's something we need to be very, very, very cognizant of. We used to have an advantage on ground game -- get out the vote, 72-hour program, all that. They now have an advantage." (The 72-hour program was an operation employed by the Republican National Committee which used aggressive voter mobilization, relying on thousands of volunteers, to drive up turnout in the final hours of an election. )
But the addition of Paul Ryan to the Republican ticket will likely help the GOP. Through Ryan, Republicans are hoping to generate enthusiasm -- the kind Democrats had in 2008 -- among tea party activists, who have shown in select congressional campaigns that they do have the ability to display muscle on the ground.
The polls have barely moved this summer and this contest remains close. While ideological vision and personal likability will be crucial, these are the three concrete tests that will likely determine who gets the biggest push toward the victory line.
Follow @CNNOpinion on Twitter.
Join us at Facebook/CNNOpinion.
The opinions expressed in this commentary are solely those of Julian Zelizer.