Mr. Romney will spend about $16.8 million on television ads this week, according to ad-tracking data provided by media buyers, while Mr. Obama will spend about $15.5 million, down 30% from a week earlier, when the campaign spent heavily to air a two-minute, pre-debate spot.

When spending by independent super PACs is included, the mismatch is wider. Mr. Romney and his allies will combine for $31.4 million this week, compared with $19.7 million for Mr. Obama and his supporters, according to data from one media tracker.

“A lot of the super PACs kind of were sitting there tepidly,” said Kyle Roberts, head of Smart Media Group, a Republican ad-buying firm. “You’re seeing them get back in the game.”