More than two-thirds of the money spent on Senate TV ads so far this cycle has been in just nine battleground states, according to a Center for Public Integrity analysis of data provided by Kantar Media/CMAG, a media tracking service.
The investigative journalism nonprofit found that candidates and outside groups have spent an estimated $153 million on more than 428,000 primary and general election ads across the country.
Candidates are only responsible for about half of these ads, while super PACs and third-party nonprofits have spent the rest.
On the Republican side, the biggest spenders have been Americans for Prosperity, Crossroads GPS and the U.S. Chamber of Commerce. On the Democratic side, the Senate Majority PAC and Patriot Majority USA have led the way.
To take control of the Democratic-led Senate, Republicans needs to win six Senate seats in November.