But Obama has a key advantage over Romney by raising the bulk of his money through his campaign committee, which qualifies for discounted ad rates under federal election laws. That can allow Obama to pay much less for the same ads compared with conservative super PACs and other outside groups, which don’t qualify for such rates.

Romney has less money under his direct control because he relies more heavily on wealthy donors who give to the Republican Party and on ad spending by well-funded independent groups. Unlike Obama, Romney also bought relatively little ad time in advance — which is cheaper — and has tended to choose more expensive ad spots with guaranteed placements.

“There was a lot of talk that Romney and his allies were hoarding resources for a major ad push in the closing moments of the campaign,” Michael Franz, co-director of the project, said in a statement. “We just haven’t seen that to date on local broadcast. And time is running out.”