Editor's note: Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" (Times Books) and of the new book "Governing America" (Princeton University Press).
Princeton, New Jersey (CNN) -- The wars over campaign spots have begun.
Last week, American Crossroads, Karl Rove's nonprofit operation that was highly effective in 2010, launched a blistering ad charging that President Barack Obama has failed to help American families.
A woman described as the mother of two grown children without jobs says to the camera, "I supported President Obama because he spoke so beautifully. He promised change. But things changed for the worse."
Obama also found himself in the middle of a controversy when Cory Booker, the popular Democratic mayor of Newark, New Jersey, criticized a spot that had attacked Mitt Romney's work at Bain Capital. It had been paid for by the Obama campaign.
Television spots are the medium through which the modern campaign is fought. The success or failure of the candidates at producing effective advertisements could have a huge influence on the outcome in November.
Each side of the campaign will spend inordinate amounts of money to pay for 30-second advertisements -- which will also be spread through the Internet -- that seek to define the message of the campaign of 2012 and the terms of the fight.
"Television is no gimmick," said Roger Ailes, Richard Nixon's campaign consultant, in 1968, "and nobody will ever be elected to major office again without presenting themselves well on it."
Ailes was right. Since 1952, when television had become a regular part of many American homes, the campaign spot has become a defining feature of modern politics.
According to The Living Room Candidate, an outstanding site that allows viewers to see many of these commercials, military hero Dwight Eisenhower launched a presidential campaign spot based on the advice of advertising executive Rosser Reeves who thought this was the best way to reach the electorate.
Democrat Adlai Stevenson was disgusted by the use of commercials. "The idea that you can merchandise candidates for high office like breakfast cereal," he famously quipped, "is the ultimate indignity to the democratic process." Eisenhower won.
Like it or not, spots have dominated campaigns. Although there have been tremendous variations in the kinds of spots Americans have seen, there have been several consistent types that politicians have used that we are likely to see in the coming months.
The first is the character assassination spot. These are the ads in which candidates exploit a perceived weakness of their opponent, highlight this weakness and use it through the spot to shape how voters perceive them. The most infamous of all was the Daisy ad in 1964: Lyndon Johnson's campaign broadcast a spot featuring a little girl picking petals off a flower until viewers saw a mushroom cloud in her eye.
The point was to take fears that Barry Goldwater was a reckless hawk on military matters and craft them into a shocking image that could scare people into believing he would launch a nuclear war.
Another famous example came in 2004, when George Bush's campaign broadcast an ad featuring Massachusetts Sen. John Kerry windsailing, moving in different directions. After going over a series of issues on which Kerry had switched his position, the narrator says, "John Kerry: whichever way the wind blows."
The ad played off perceptions that Kerry lacked core principles and sold this as the essence of his character. A more vicious ad in that campaign came from an independent group that featured veterans questioning Kerry's war record during Vietnam. The ad gave rise to the term "swift-boating" in modern campaigns.
Another type is the issue-based spot, which highlights a particular area of controversy that does not play well for the opposition or which triggers a debate about a subject that will raise problems for opponents.
One of the first ads by Eisenhower, called "High Prices," featured him answering questions to average citizens (who were filmed separately though it looked like they were addressing him), many of which revolved around the cost of living and inflation.
President Richard Nixon broadcast a powerful ad in 1972 that featured plastic soldiers being wiped off the table as a way to raise fears about defense budget cuts taking place under the Democratic Congress and which would accelerate if Sen. George McGovern was in the White House.
In 1988, campaign guru Lee Atwater filmed the "Willie Horton" ad, which revolved around a felon furlough program in Massachusetts when Michael Dukakis was governor, to talk about law and order.
There are also the guilt by association spots through which candidates try to tie an opponent to an unpopular figure or events through the power of image. In 2008, Barack Obama ran an ad that showed Republican John McCain embracing the unpopular George W. Bush repeatedly. In one of Richard Nixon's ads in 1968, the camera showed images of Vice President Hubert Humphrey juxtaposed with the chaotic Democratic Convention in 1968 where anti-war protesters clashed with the city police in Chicago's Grant Park. No words were needed.
The final spot is a very different kind, the candidate-boast spot. It is usually simple and direct, literally an advertisement for the person who is running. Jimmy Carter broadcast a very effective example in 1976, when he talked about his personal story in Georgia and used his own upbringing as the most important characteristic of the campaign. "1976. Across our land," the narrator says, "a new beginning is under way, led by a man whose roots are founded in the American tradition."
Ronald Reagan's campaign featured "Morning in America" in 1984 which highlighted the revival of the economy, and emphasized that the nation would be strong. "Under the leadership of President Reagan, our country is prouder and stronger and better." In 1996, Bill Clinton's campaign spoke about his being a bridge to the future.
This year both campaigns will likely draw on all of these kinds of spots to sell their message. They will be the subject of debate and play an important role in shaping the perceptions that voters have of each candidate.
Obama and Romney will need to be careful that the spots they broadcast, or which independent groups broadcast for them, don't backfire on them rather than their opponents, and they will need to make sure to convey enough positive messages along with the negative. But the candidate that pulls off the best ad campaign will vastly improve his odds of winning the White House in November.
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The opinions expressed in this commentary are solely those of Julian Zelizer.