Inside the costliest race of the most-expensive congressional election ever

The congressional elections being held in six days will go down in history as the most expensive ever.

More than $4 billion has been spent on behalf of candidates, and that's just the money we know about.

The costliest race is the Senate battle in North Carolina, where spending topped $100 million this week.

What does all that money buy?

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Sen. Kay Hagan, D-N.C., left, and North Carolina Republican Senate candidate Thom Tillis shake hands prior to a live televised debate at UNC-TV studios in Research Triangle Park, N.C., Tuesday, Oct. 7, 2014.
AP Photo/Gerry Broome, Pool

Democrat Kay Hagan and Republican Thom Tillis are all smiles on the stump. But voters here endured 10,800 negative ads last week.

Two-thirds of them were paid for by outside groups.

"North Carolina is not for sale!" Hagan told voters at a recent event.

As a Democrat in a battleground state, Hagan became the target of conservative ads a full year ago. Democratic groups rushed in to even the score.

"You know I wish that we had, together, said no outside money," Hagan says. "And I tell ya, I think it would have been a different day."

The Supreme Court paved the way for unlimited outside spending in a 2010 decision commonly known as Citizens United. It overturned parts of a campaign finance law authored by Republican John McCain, who we caught up with at an event for Tillis in Charlotte.

McCain says at this point, there's no way to put the genie back in the bottle.

"Not until there is a scandal," McCain says. "I'm sorry to tell you that. There just doesn't seem to be the interest -- both sides enjoy having all this money.

But Tillis, who is speaker of the state House, says the money drowns out his message.

"If you believe some of the ads, you would think that Sen. Hagan was a bad person and I was too," Tillis says. "I hope that most of the voters will go out take a look at the real issues, what I stand for, what I have done in North Carolina, and what Sen. Hagan has failed to do in Washington."

The irony is that despite all that spending, the polls in North Carolina have barely budged. According to our Battleground Tracker, Hagan had a one point lead over the summer and leads by three points now.

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    Nancy Cordes is CBS News' congressional correspondent.