Super PACs and tax-exempt groups are increasingly fueling the cost of campaigns as candidate and party spending is shrinking, according to a new analysis by the Center for Responsive Politics, a nonpartisan research center.

While the group now expects the cost of the 2014 midterms to be on par with the 2010 elections, an increasing share is being fueled by independent organizations that are not subject to fundraising limits.

CRP projects that the tab for 2014 will be at least $3.67 billion, down from the nearly $4 billion the group originally estimated it would cost. Super PACs and other independent political players are expected to account for nearly $520 million, up from $309 million in 2010. Candidates are expected to shell out $1.58 billion, down from $1.8 billion in 2010.

The totals do not take into account tens of millions of dollars that were spent this year by tax-exempt groups on issue advocacy, which does not have to be reported to the Federal Election Commission.

Even with all that activity, the volume of ads this year is down slightly from the last midterms, according to a new analysis of Kantar Media data by the Wesleyan Media Project. That’s large part because TV spending on House races has plummeted compared to 2010. Instead, the air war has been concentrated on the fight over control of the Senate, with nearly 400,000 spots airing since Sept. 1, up 12 percent over fall 2010.

Despite a last-minute surge in spending by pro-GOP groups, Democrats and their allies have maintained an advantage on the airwaves in the Senate battle, airing about 30,000 more spots than Republicans and their backers this fall.