Perhaps the best symbol of this election is not the logos of the presidential candidates, nor the Republican elephant or the Democratic donkey. A better emblem would be a very, very large pile of cash.

One of the most closely fought elections in U.S. history is also its most expensive. The final price tag will reach $6-billion (U.S.), eclipsing the 2008 campaign by roughly $700-million, estimates the Center for Responsive Politics, a non-partisan research group.

Whether the advertising and electioneering bought with this mountain of money has swayed voters or annoyed them remains an open question.

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One big force driving spending higher: the arrival of new outside groups that can accept unlimited donations and spend the funds on political ads, so long as they don't co-ordinate with a candidate's campaign. Such groups, which include so-called "super" political action committees and non-profit organizations, are on track to pump close to $1-billion into the election.

Despite their anodyne names – "Restore Our Future," say, or "Priorities USA Action" – such groups have crystal-clear political goals (the former super PAC is pro-Mitt Romney, and latter is pro-President Barack Obama).

Such outside groups have brought new opacity to campaign finance. "It's the Russian doll problem," said Ciara Torres-Spelliscy, a law professor who specializes in campaign finance at Stetson University in Florida. Super PACs must disclose their donors, but if the donor is a non-profit organization, then that group doesn't have to reveal its sources of funds, and the trail ends there.

Alongside the endless campaigning of recent months, the candidates have engaged in a parallel endeavour of epic fundraising. In the end, it appears that the Republican drive to put Mitt Romney in the White House has spent more money than the Democratic effort to re-elect Mr. Obama, but not by a huge amount. Neither presidential candidate accepted public funding for his general election campaign, the first time that has occurred since the program was set up in 1976. That's because such public money comes with strings attached in the form of spending limits.

Out of political necessity, candidates "have to spend more and more time raising money," said Brendan Doherty, a professor at the U.S. Naval Academy and author of The Rise of the President's Permanent Campaign. By Mr. Doherty's calculations, Mr. Obama has held 220 fundraisers focused on his re-election during his first term. That's more than double the number held by his four immediate predecessors at the same point in their tenures.

Of course, an election is not an auction, to borrow the words of a Connecticut politician who was vastly outspent by his opponent two years ago but triumphed nevertheless. It's far from clear that this sea of money is having the desired effect.

In critical swing states and in crucial races, the influx of funds – sometimes from shadowy groups specializing in nasty ads – may make voters stop listening. "You crank up the volume and you crank up the lies and deception," said Edwin Bender, executive director of the National Institute of Money in State Politics. "People are starting to recognize it."

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Romney's Money Man

Sheldon Adelson:$53-million

Goal:A billionaire Las Vegas casino magnate, Mr. Adelson has written that he believes Republicans are more staunch supporters of Israel and better stewards of the economy. Contributions by him and his wife to conservative outside groups include $20-million to Restore Our Future, a super PAC backing Mr. Romney. For context, that single contribution is roughly the same as the spending limit for the Liberal Party in Canada for the country's national election in 2011.

Obama's Money Man

Jeffrey Katzenberg:$2.6-million

Goal:The hard-charging chief executive of DreamWorks Animation SKG Inc., Mr. Katzenberg threw his support behind Mr. Obama in 2008. This time around, he has contributed to the President's campaign, arranged for friends to give to the effort, and also donated $2-million to Priorities USA Action, a pro-Obama super PAC.The entertainment industry remains a dependable source of funds for Mr. Obama, together with trial lawyers. Some sectors which supported Mr. Obama four years ago – notably the financial industry – have deserted him in this race.

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The Biggest Super PAC

Restore Our Future:$143-million

Goal:Launched by former Romney aides, Restore Our Future is at the vanguard of the race to inject more money into the American election. While it is pro-Romney, it is not supposed to co-ordinate with the candidate's campaign. Among the 26 donors to the group who have given $1-million or more, 11 are in the hedge fund or private equity business, according to the non-profit Center for Public Integrity. Restore Our Future's $1-million-plus club also includes Texas home builder Bob Perry and Oxbow Corp., an energy company run by William Koch, the lesser known of the three Koch brothers. (the other two are Charles and David, both prominent supporters of conservative causes).

The Biggest Corporate Contributor

Specialty Group Inc.:$5.3-million

Goal:The news that this company is the largest corporate contributor to the election campaign may prompt some head-scratching. Specialty isn't exactly a household name. In fact, it may not even be a functioning business. According to the Center for Public Integrity, the company was formed last month and gave $5.3-million to FreedomWorks for America, a super PAC supporting Republican candidates (its website urges Americans to "fire Obama"). Interestingly, it's possible that there are other companies that have given more than Specialty – but they may have done so through non-profit groups, which don't have to disclose their sources of funds.

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Sources: Center for Responsive

Politics, Associated Press