Midterms likely to be most expensive ever as TV ads pass billion-dollar mark

Campaigns, political parties and outside groups have booked or aired more than $1 billion in television advertising, months before voters head to the polls in November in what is likely to become the most expensive midterm election campaign in U.S. history.

In the fight for control of the U.S. Senate, where Republicans hold a narrow two-seat majority, the two sides have already spent a combined $170 million on television spots, according to an observer keeping close tabs on the advertising market. 

The parties have another $230 million in airtime booked in key Senate races. 

Democrats hoping to win back control of the House have aired or booked more than $135 million in television airtime, while Republicans playing defense have spent or reserved $146 million, numbers that will almost certainly increase as prominent outside groups raise and spend more money.

And the two sides have reserved a whopping $340 million in airtime in gubernatorial races across the country. The majority of that money, $250 million, has already been spent on advertisements ahead of key primary elections, hinting at an ad onslaught set to heat up in the weeks after Labor Day.