Outside groups have deployed half a billion dollars in an effort to sway the 2012 election, and more than three-quarters of that total can be traced to the Supreme Court's Citizens United decision.

The landmark Citizens United v Federal Election Commission ruling, handed down in 2010, lifted caps on how much outside groups can spend on elections, by deeming that type of activity a constitutionally protected form of free speech. The result has been a proliferation of entities, notably Super PACs, that can pour unlimited sums of money into advertisements, mailers and other electoral activities.

According to a new analysis by the nonprofit Sunlight Foundation, the 2012 cycle has so far attracted $465 million in independent expenditures. 78 percent of that total flows from relaxed spending limits due to the Citizens United decision, the Sunlight Foundation found.

$272 million of the total came from Super PACs, groups that came into being after the Citizens United ruling. Super PACs are free to spend endless amounts of money as long as they do not coordinate their activities with campaigns, although that partition has proven to be a flimsy one.

Super PACs have become an indelible feature of the political landscape, allowing donors to funnel vast amounts of money into political advertisements. After initially decrying the Citizens United decision and seeking to eschew Super PACs, President Obama reversed course earlier this year and urged donors to begin giving to the Obama-aligned Priorities USA Super PAC.

Another $93 million of the $465 million total comes from groups that are designated as nonprofits or trade groups, a designation that puts them outside the purview of the Federal Elections Commission and allows them to spend freely without reporting the source of their donations. This category encompasses organizations like Crossroads GPS and the Chamber of Commerce.

Those groups can only keep their special tax status if the group's primary activities are issues-based, rather than directly encouraging voters to vote for a certain candidate. That rule, too, is easily skirted, and the Obama campaign filed a complaint earlier this year charging that Crossroads GPS is clearly a political organization and must disclose its donors.

The combined $365 million is more than double the amount that such groups spent on the 2010 campaign, the Sunlight Foundation found, but the increase does not appear to be solely attributable to the fact that 2012 is a presidential election year. Most of the spending by outside groups this time around is focused on Congressional races.

That reflects the prominent role outside groups have played in Congressional contests this year, notably Senate primaries in states like Texas and Indiana, where outside spending has helped more conservative candidates unseat Republicans with establishment backing.