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."...

Like it or not, [TV] 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....

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....