A full 86 percent of Obama’s television advertising and 79 percent of Romney’s has been negative, according to the Wesleyan Media Project, which tracks political advertising. By comparison, Obama and John McCain had spent an average of 69 percent of their TV budgets on negative ads by this point in 2008, and George W. Bush and John Kerry had spent 58 percent in 2004.

The record-setting rancor reflects the circumstances of the 2012 race: Obama couldn’t run on an economic rebound, and some of his biggest legislative accomplishments, such as his health care law, are unpopular in the polls. Romney has tried to persuade the American people to fire the incumbent — but has been light on details of his own.

“The dynamic of this election is such that each campaign wants the race to be a referendum on the other guy, and therefore the mass of the communications is designed to disqualify each other,” said Mark McKinnon, George W. Bush’s former ad man.