Why These Are The Most Expensive Midterms Elections Ever

The total cost is expected to run up to $3.67 billion

The bottom line for this year's races is going to come down to $3.67 billion, which the Center for Responsive Politics reports is the most expensive midterm campaign ever.

That total nudges past the last midterms in 2010, which CRP reports cost $3.63 billion, but experts believe it's the way in which the costs were increased that is more important than the total number.

"To me, what's most troubling about all this is not just that there's been an increase, but that we don't know who is behind the spending," Lawrence Norden, a deputy director for New York University's Brennan Center for Justice, told ABC News.

This election cycle has been a particularly big one for such dark money groups, Norden said. Dark money groups will have spent nearly $200 million in the 11 most competitive Senate races this year, which is more than double the $97 million they spent in 33 competitive Senate races in 2012.

"From what we've seen from the past few elections is that these trends keep getting more and more extreme," Norden said. "That there's more and more outside money and more and more of it keeps coming from unknown sources.

For Brian Baker, the president of conservative Super PAC Ending Spending Action Fund, the Supreme Court ruling made spending a part of the constitutional right to free speech, and should be protected as such.

"I'm a huge huge fan -- a big believer -- in the Constitution," Baker told ABC News.

"There are groups in the political space that are exercising their First Amendment rights and there are other groups and, in the process of exercising their First Amendment rights, they're trying to take away mine," he said.

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