Ad purchases in the presidential race doubled or in some cases tripled last week in swing states such as Colorado, Florida, Iowa and Virginia, tracking data show. The surge is being driven by Romney and well-funded allies, who decided against running more ads earlier in the campaign in favor of a big bang at the end.

Restore Our Future, a super PAC dedicated to helping Romney, has booked $14 million worth of ads in nine states for the final week of October — more than it spent on ads during the month of September. The group is also ramping up its spending, airing a mix of ads criticizing Obama and extolling Romney in Florida, Iowa, Michigan, Ohio and Virginia…

The ramped-up advertising by Republicans left Obama behind his GOP foes in total ad expenditures last week for the first time since the summer, though he has massive cash reserves after raising $181 million in September. Obama and his key outside ally, the Priorities USA Action super PAC, have kept up a steady barrage ads attacking Romney in Ohio and other battlegrounds…

Brad Todd, a Republican media strategist, said he suspects that the big push at the end is designed to reach voters displeased with Obama but unwilling to embrace Romney — fence-sitters who have delayed making up their minds.

“Advertising at the end typically makes the biggest difference to those voters,” Todd said.