Outside spending benefiting Obama over the same period never exceeded $14 million, records show.

Super PACs have raised about $660 million in the 2012 election.

Restore Our Future alone accounted for about $1 out of every $5 in super PAC donations received. The pro-Romney group raised more than $130 million, much of which was spent decimating the candidate’s rivals during the Republican primary.

The Obama-backing Priorities USA Action raised $64 million.

The amounts have grown significantly: In 2010, their first year of existence, all super PACs combined raised $85 million.

The explosion in outside spending has coarsened the political debate, flooding the airwaves in Ohio, Florida, Virginia and other battleground states with negative, often inaccurate ads.

Roughly 80 percent of all spending by conservative groups and liberal groups has been negative, FEC records indicate.

All of the nearly $57 million that Priorities USA Action reported spending has been on negative ads about Romney. The group, which coined the slogan “If Mitt Romney wins, the middle class loses,” linked him to the death of a woman who lost her battle with cancer.

Another of the super PAC’s most memorable ads featured a worker describing how building the stage on which officials announced his plant’s closure, after it was bought by Bain Capital, which Romney helped found, was like building his own coffin and it made him “sick.”

Eighty-eight percent of Restore Our Future’s spending went toward negative ads, as did 95 percent of the expenditures of American Crossroads, another super PAC.

Many of these ads have criticized Obama’s handling of the economy, arguing that the country “can’t afford” four more years of the president’s policies. One spot features a small-business owner saying, “We can’t create more jobs until Obama loses his.”

Others ads have featured disillusioned Obama supporters from 2008 expressing disappointment with the president.

The top 149 individual super PAC donors – each of whom has contributed at least $500,000 – are responsible for $290 million of the money raised.

And 858 individuals who contributed at least $50,000 to super PACs accounted for nearly 60 percent of the money the groups have collected in the 2012 election. By way of comparison, the median household income in 2011 was $50,054, according to the Census Bureau.

Donations from large, publicly traded corporations have been relatively rare, but in the waning weeks of the campaign, oil and gas giant Chevron wrote a $2.5 million check to the Congressional Leadership Fund, a super PAC backing Republican candidates that’s closely associated with House Speaker John Boehner, R-Ohio.

The emergence of super PACs has been heralded by some, such as Republican lawyer Brad Smith, a former chairman of the Federal Election Commission who co-founded the conservative Center for Competitive Politics.

Super PACs “have helped to level the playing field between Romney and Obama, whereas otherwise Obama’s spending advantage would have been substantial,” Smith said. “And in some cases they have raised issues that concern voters that the candidates have chosen to avoid.”

Others disagree.

“When elected officials rely on the most-wealthy of wealthy Americans, it means the voices of everyday people lose out,” said Nick Nyhart, the president of the advocacy group Public Campaign, which favors publicly financed elections.