WASHINGTON - Accelerating spending by outside groups in the final weeks of the 2012 campaign will drive total spending on federal elections this cycle to a record $6 billion, said an analysis by the nonpartisan research group Center for Responsive Politics.

That tops the record set in 2008 by $700 million, making this campaign the most expensive in U.S history.

The major force behind the spending: hundreds of super PACs and advocacy groups, which proliferated after a series of federal court rulings in 2010 lifted the ban on corporate political activity and permitted wealthy donors to pool unlimited sums of money for election spending.

"We are in an entirely new environment," said Sheila Krumholz, the group's executive director.

As of Wednesday, outside groups had reported spending more than $892 million on TV ads and other forms of voter contact. In all, CRP estimates that expenditures by outside groups will reach more than $970 million this election. That's more than three times the previous record of $301 million in 2008.

The total does not include millions of dollars spent by tax-exempt advocacy groups on issue ads and other forms of voter outreach that do not have to be reported to the Federal Election Commission. Those groups also do not have to report the sources of their funds.

"It is confounding, because we have no idea how much we're really seeing," Krumholz said. "We know it's some part of the iceberg."

More than half of the money spent by outside groups has gone to efforts to influence the presidential race -- spending that has picked up as Election Day approaches. In September, super PACs and advocacy groups spent $19 million a week on their presidential efforts. By week of Oct. 21, they were up to $70 million a week.

Republican Mitt Romney has largely been the beneficiary of the outside groups, with more than three-quarters of the money spent on his behalf. Overall, this year's presidential campaign spending is expected to be $2.6 billion, down slightly from $2.8 billion in 2008, when both parties had contested primaries.