“The most fundamental point about political advertising is that it matters at the margins,” said Erika Franklin Fowler, director of the Wesleyan Media Project, which tracks and analyzes campaign ads. “It might help in a close election,” but factors such as the state of the economy and partisan identification are much more influential, she said. …

The risk of losing even a fraction of the vote by being outspent on advertising largely drives the relentless fundraising and spending of both parties, says Travis N. Ridout, a political scientist at Washington State University. “You don’t want to be left behind,” Ridout said. “No one is willing to unilaterally disarm.” …

Local TV advertising will consume about half of all the dollars raised by the candidates, their parties and the independent super PACs that are the wild card in this year’s campaign, Goldstein estimates. That means at least $1 billion will be spent on TV spots in just a dozen or so swing states that could determine the election. Others guess more — lots more: Bill Burton, a former aide to President Obama, told New York magazine last month that the total raised this year could top $2.6 billion, with much of that total ticketed for advertising.