A word of warning to swing-state voters who have suffered through an onslaught of attack ads this summer: The worst is yet to come.

Federal candidates and their supporters are gearing up to unleash up to $3 billion worth of advertising and other expenditures over the next nine weeks, drowning battleground areas in political ads and setting loose legions of canvassers aimed at getting out the vote on Nov. 6.

In the presidential contest alone, President Obama, Republican nominee Mitt Romney and their allies are poised to spend well over $1 billion from now to November, much of it focused on the handful of swing states that are likely to decide the election, according to a Washington Post analysis of campaign finance reports and other data.

The frenzy is likely to make 2012 the most expensive election in U.S. history, due in part to a new breed of super PACs and nonprofit groups that can raise unlimited funds for elections. The Center for Responsive Politics, a nonpartisan group that tracks money in politics, estimates that federal campaigns and their supporters will spend nearly $6 billion in the 2012 cycle, surpassing the $5.4 billion watershed reached in 2008.

And for the first time since the 1970s, neither major presidential candidate is constrained by the limits of public financing.

“There’s going to be more intensity at the end than previous cycles, if for no other reason that Republicans will have more resources than last time,” said CRP senior fellow Bob Biersack, who analyzed campaign data for decades at the Federal Election Commission. “The ads are already just laying on top of each other in the swing states. You wonder how much impact the messages can have when they come in sets of four or five in a row.”

The Obama and Romney camps have each spent well over $400 million so far through political committees, parties and independent groups, including money that Romney devoted to securing the GOP nomination, according to FEC and advertising data.

But Romney now commands a sizable lead in cash on hand and will be able to rely on help from well-funded conservative groups such as American Crossroads, which has said it plans to spend $200 million on the presidential race alone through its nonprofit and super PAC arms. Taken together, super PACs and other independent groups could easily end up spending over $1 billion on federal races, most of it in favor of Republicans, according to various estimates.

Tim Phillips, president of Americans for Prosperity, said the conservative group will devote at least $100 million to all its activities this year, including more than $50 million already spent on advocacy and issue ads in 18 battleground states. The group also has a 100-person field operation, he said.

Phillips said this fall will look dramatically different for Democrats compared with 2008, when Obama enjoyed a clear financial advantage over Sen. John McCain (R-Ariz.), whose fundraising ability was limited by his acceptance of public matching funds for the general election.

“Normally a sitting president does have a substantial financial edge, so it’s unusual in that respect,” Phillips said, but he added, “It only matters to the outcome if you’re heavily outspent, and I don’t think either side will be heavily outspent.”

In the summer of 2008, Obama actually had a bit less cash on hand than McCain and the Republicans but then surged ahead once the general election began. History is unlikely to repeat itself this time, however: Romney began August with a $60 million financial edge on Obama and can also rely on deep-pocketed outside groups to help him.

Most strategists expect that Romney and the outside groups aligned with him will dominate in broadcast advertising from here on out, while the Obama campaign banks on a 3-to-1 staffing advantage in swing states to bring out voters.

Bill Burton, a spokesman for Priorities USA Action super PAC, said his group can still be effective while being outspent by carefully targeting its ads, which will total about $30 million from now to Election Day. He pointed to Priorities ads over the summer that focused national attention on Romney’s tenure at the Bain Capital private equity firm.

“They broke through more than anything else and had an impact on how people think about Mitt Romney’s business record,” Burton said. “Money is important. But I don’t think we need to spend as much as them to have meaningful impact.”

The brunt of the fall political ads will be focused in just over a half-dozen swing states that have already borne most of the assault so far, including Colorado, Iowa, Florida, Nevada, North Carolina and Virginia. States with hotly contested Senate races will also be hard hit, experts say.

Ken Goldstein, president of the Campaign Media Analysis Group, which tracks political ads, said that “for people in those battleground states and battleground markets, it’s going to be all campaign ads all the time.”

Although many voters are likely to tune them out, the spots are primarily aimed at a tiny slice of the electorate that remains undecided in a polarized election year. Goldstein compared it to an occasional sports fan who only takes notice when the local team makes the playoffs.

“If you’re a casual fan, if you’re someone who only watches the Super Bowl, you’re only going to start tuning in at the end of the season,” Goldstein said. “These ads aren’t trying to convince everybody. The thing about people who are undecided is that they are probably not following politics yet.”

T.W. Farnum contributed to this report.