Nearly $1 billion worth of political ads have inundated the airwaves ahead of this year's midterm elections, driven in part by a stronger Democratic presence on the airwaves than in 2010, according to a new study by the Wesleyan Media Project.

An estimated $337 million has already been spent to air more than 728,000 ads in Senate races, surpassing the $324 million spent during the 2012 elections on Senate campaigns, according to Kantar Media/CMAG data analyzed by Wesleyan. The project examines buys on broadcast television and national cable, but not local cable.

The largest share of TV spending this year has been on governor’s races, with an estimated $426 million spent so far.

“We’re definitely going to hit $1 billion before election day,” said Travis Ridout, co-director of the Wesleyan Media Project.

Cities such as Denver, Tampa and Little Rock -- including both Senate and governor’s races -- are seeing the most intense volume.

“It seems like presidential-year levels in some of these races,” Ridout said.

In Senate races, Democrats continue to enjoy an edge in the air war, in part because a larger share of pro-GOP ads have been run by outside groups, which must spend a higher rate than candidates. In Louisiana, for example, 4,682 pro-Democratic ads aired during the last two weeks, compared to 3,129 pro-Republican ads.

The Wesleyan analysis illustrates how Democrats have had a bigger TV arsenal than they did in 2010 Senate races. Between Sept. 26 and Oct. 9, 32,929 pro-Democratic ads aired -- an increase of 34% over the same period four years ago. Meanwhile, pro-GOP ads in Senate races were down 8.5% for the same period.