Most of the attention heading into Election Day may be on the presidential race, but the stakes are also high in the battle for the U.S. Senate, where there are close contests in about a dozen states.
According to an NPR analysis of Kantar Media CMAG data, outside groups are spending more than $100 million blanketing the airwaves. This won't come as a surprise if you live in a state with a competitive Senate race.
Take Ohio, the Senate race with the most TV ad spending. As of last Saturday, broadcast stations had aired more than 64,000 ads, but the majority didn't come from the candidates or the parties they came from outside groups.
Conservative groups such as Crossroads GPS and the Club for Growth have both run ads criticizing Sen. Sherrod Brown, the Democratic incumbent. While the National Education Association's PAC, the NEA Fund, has aired ads attacking the Republican candidate, Josh Mandel.
According to the Kantar Media CMAG data, groups supporting Mandel have run four times more ads than those backing Brown.
"We are being significantly outspent," says Guy Cecil, executive director of the Democratic Senatorial Campaign Committee. "What we've seen really over the last year is an unprecedented amount of money, largely being spent by a dozen billionaires from around the country who are willing to spend just about anything in order to win."
In reality, it's hard to know where the money is coming from.
On the Republican side in Ohio, more than 95 percent of the independent TV ad spending has come from groups not required to disclose the identities of their donors. This includes Crossroads GPS, the biggest spender in Ohio and in Senate contests nationwide. The U.S. Chamber of Commerce is a not-so-distant second.
"We're up there. That's good. I'd be disappointed if we weren't," says Scott Reed, the Chamber's senior political strategist.
As of last Saturday, the Chamber had spent more than $5 million on TV ads in Ohio alone, one of 15 Senate races where its ads were on the air.
Reed describes the Chamber's efforts this election year as "the largest most aggressive voter-education campaign in our 100-year history."
He says the fact so much of this spending is coming from groups with secret donors is free speech.
"The chamber is a 501(c)(6) organization that operates legally," Reed says. "We do not have to disclose our donors."
Many argue the Ohio Senate race wouldn't even be competitive without all of the outside spending to boost the Republican challenger.
On the airwaves, Mandel, Ohio's treasurer, only spent a third of the $18 million spent by these independent groups.
David Keating, president of the Center for Competitive Politics and the father of the superPAC, isn't persuaded.
"The fact that a race may be competitive when it wasn't expected to be? I think we should be celebrating that," Keating says.
When it comes to TV spending by outside groups, the split has been far from even. Republican candidates have benefited more than twice as much as their Democratic counterparts.
In a majority of those races, like in Ohio, the Republican candidate's campaign was actually outspent by outside groups.
In Virginia, former Sen. George Allen has spent less than $3 million on TV, while the outside groups supporting him spent more than $14 million.
In contrast, just two of the 10 Democratic candidates were outspent on TV by outside groups that backed them.
And an even starker partisan divide? More than 80 percent of all the Republican outside money comes from secret donors. On the Democratic side, less than 10 percent of the money is secret.
But, money isn't everything. Many political forecasters say they expect Democrats will maintain control of the Senate, something that a year ago seemed nearly impossible.
Cook Political Report's Jennifer Duffy says Republicans were hurt by a surprise retirement and controversial comments on rape by Senate candidates Todd Akin of Missouri and Richard Mourdock of Indiana.
"There is not a thing that millions of dollars of outside group advertising could have done to change those situations," Duffy says.
Still, every one of these groups would likely say, it was worth the effort and the money no matter the outcome.