The 2012 election is officially the most expensive in U.S. history, estimated to reach $6 billion thanks to massive amounts of spending by groups that -- at least officially -- are not connected to campaigns or political parties.

Spending for this election cycle has surpassed the next most expensive election by more than $700 million, according to the Center for Responsive Politics, which credits the gap to the unprecedented amount of money being raised and spent by outside organizations. While earlier this year the nonpartisan organization estimated the 2012 election would cost approximately $5.8 billion -- a prediction that still would have made it the most expensive in American history -- an analysis of the most recent Federal Election Commission data forced the group to revise that number, with less than a week to go until Election Day.

“In the new campaign finance landscape post-Citizens United, we're seeing historic spending levels spurred by outside groups dominated by a small number of individuals and organizations making exceptional contributions," Sheila Krumholz, the executive director of the Center for Responsive Politics, said in a statement.

The increase has been largely driven by super PACs and other outside groups that can raise unlimited amounts of money from donors, the result of the U.S. Supreme Court’s 2010 Citizens United decision. Because these organizations are permitted to raise unlimited funds from corporations, unions and individual contributors to influence political elections, the CRP reports spending by those groups could surpass $970 million this election cycle.

And that may be a lowball estimate. While outside spending groups spent at least $539.4 million through Wednesday, definite estimates are difficult to gauge because the rate of spending by those groups has been skyrocketing in the closing weeks of the election. A prime example is the presidential race, where spending has grown from $19 million per week (for ads for and against the two main candidates) in early September to an astonishing $70 million during the week beginning Oct. 21.

But there is still hundreds of millions of dollars of election spending that government transparency groups cannot account for. Often referred to as shadow money groups,  these nonprofit organizations can also raise unlimited sums of money for purportedly educational (not political) activities and are not required to register with the FEC. These groups have spent at least $203 million in the last two months -- and it will likely never be known where the vast majority of that money came from.

At least another $100 million has been spent by these groups on “issue advocacy” advertisements that actually identify a federal candidate but still are not required to report to the FEC. According to the CRP, this practice is concerning because three of the top six outside spending groups this election cycle are shadow money groups. That means voters are being exposed to a countless number of political advertisements with absolutely no way of judging the credibility of their messages.

The massive amount of outside spending isn’t limited to national politics. Between 2008 and 2012 (neither of which included midterm elections), outside expenditures in the U.S. House and Senate rose from $46 million to $455 million -- a tenfold increase suggesting that big spenders, and not constituents, are gaining a stronger hold over elected officials at each end of the spectrum.