WASHINGTON (AP) — The 2012 presidential campaign passed the $2 billion mark in fundraising Thursday, fueled by an outpouring of cash from both ordinary citizens and the wealthiest Americans hoping to influence the selection of the country's next leader.

The eye-popping figure puts the election on track to be the costliest in modern U.S. history. It comes amid a campaign finance system vastly altered by the proliferation of outside groups and "super" political committees that are bankrolling a barrage of TV ads in battleground states.

President Barack Obama and Republican challenger Mitt Romney have brought in about $1.7 billion so far this election, according to fundraising reports submitted Thursday night.

Added to that: nearly $300 million in donations involving super PACs since early 2011, as well as tens of millions more in donations to nonprofit groups that run election-related ads but don't have to disclose their donors.

Obama, the Democratic Party and related fundraising committees raised a combined $88.8 million for the first 15 days of October, reports showed, while Romney's fundraising apparatus reaped $111.8 million during the same period.

The largest of those were two pro-Romney groups. American Crossroads, a Republican-leaning super PAC with ties to former President George W. Bush's longtime political counselor Karl Rove, reported raising at least $79.6 million through Oct. 15. Restore Our Future, founded by former Romney aides, reported pulling in $130.6 million so far. And Priorities USA, a pro-Obama group founded by two former aides to the president, reported $62.8 million in contributions.

Added to that: nearly $300 million in donations involving super PACs since early 2011, as well as tens of millions more in donations to nonprofit groups — often affiliated with super PACs — that run election-related ads but don't have to disclose their donors.

Presidential candidates in 2008 raised more than $1.8 billion in inflation-adjusted figures. This time, new factors have contributed to the escalation in the campaign money chase.

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This year marked the first time that both major party candidates opted out from the public financing system established to set limits on how much a presidential candidate can raise and spend. Both Obama and Romney would have been eligible for about $100 million in taxpayer money to support their campaigns through the general election, but both gambled — correctly — that they could raise and spend far more.

In 2008, Obama became the first presidential contender to refuse all public financing while his Republican rival, Arizona Sen. John McCain, accepted the government funds. The lopsided result — Obama outspent McCain by more than 2-to-1 in the general election — effectively ended public funding as an option for serious candidates.

With the 2012 election so tight, both Obama and Romney have spent considerable time at high-dollar fundraising events courting wealthy donors. Romney last month lamented the time spent fundraising rather than speaking to larger groups of voters, saying that "fundraising is a part of politics when your opponent decides not to live by the federal spending limits."

Both Obama and Romney have raised considerable cash from small donors, too, especially the president. His campaign has reported that more than 4 million donors have contributed.

Obama spokesman Adam Fetcher acknowledged Thursday that Romney and his supportive super PACs were outspending the president on the airwaves. He said the Obama campaign was making efforts to expand its donor base as it headed into the remaining days before the election.

Federal election regulators have raised the limit on individual contributions to candidates, which means campaigns can solicit more money from donors than they have in the past. Individual donors can now give a total of $5,000 in the primary and general elections to a candidate, compared to just $2,000 in 2000.

Michael Toner, a Republican campaign finance lawyer and former chairman of the Federal Election Commission, said the close race between Obama and Romney and the sharply polarized electorate have also played a role in accelerating the dash for dollars.

"I don't know any campaign manager who thinks they have too much money. In this political 50-50 environment you can't ever have enough," Toner said. "Every last million could make the difference in who is elected."

But the emergence of super PACs and other outside groups, emboldened partly by the Citizens United decision by the Supreme Court in 2010, has done more than anything else to reshape the contours of presidential campaign fundraising. A handful of federal court cases have broadly eased campaign finance regulations, allowing donors to give unlimited sums. That kind of money has largely been funneled to super PACs, which can raise and spend money on behalf of candidates as long as they don't coordinate expenditures or strategy with the campaign.

"The distinctive factor in this election is the outside money being spent and the corrupting money financing it," said Fred Wertheimer, a longtime campaign finance reform advocate. "It's a symbol of the disastrous campaign finance system we have and the undue influence relatively few well-financed individuals and interest groups now have over government decisions.

Las Vegas casino mogul Sheldon Adelson is the top super PAC donor this year. Adelson, a billionaire, has contributed more than $40 million to Republican super PACs, including those backing Romney and former candidate and House Speaker Newt Gingrich.

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Fouhy reported from New York.

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