Thirty-six Senate races, $3.6 billion in total costs and one election: Here's a look at 2014 midterm spending, by the numbers. (Julie Percha/The Washington Post)

The record amount of independent spending swamping this year’s congressional races is fueling a deeply negative political atmosphere, as attack ads from independent groups overwhelm the messages of candidates in many competitive races.

More than 80 percent of the money spent by big-money independent groups and parties in the general election has gone to oppose a candidate, according to a Washington Post analysis of campaign finance data collected by the Center for Responsive Politics, a nonpartisan research organization.

The biggest target: Thom Tillis, the GOP Senate candidate in North Carolina, who has already endured a $26 million offensive in what is expected to be the costliest Senate race in U.S. history.

Overall, the 2014 House and Senate races are attracting record levels of non-candidate spending, with $594 million spent by independent groups and political parties so far this cycle — more than every previous federal election other than the 2012 presidential contest, according to the center.

The volume in the final days is staggering: In one seven-hour period Wednesday, independent groups and parties reported spending $17 million on congressional races.

This year’s midterm races have seen a record amount of outside spending.

The biggest drivers of this year’s frenzy of ads and voter calls have been players such as super PACs, politically active nonprofit groups and unions that operate outside traditional campaigns. Such groups have already plowed at least $428 million into messages and outreach; party committees have spent $166 million.

Those tallies underestimate the full amount being invested in House and Senate races. Tens of millions more have been spent by tax-exempt groups that keep their donors secret. Such organizations report to the Federal Election Commission only what they spend on direct political appeals — activities that have totaled nearly $135 million, more than in any other congressional election.

“Even more concerning than there being so much money in outside groups is how much of it is hidden money,” said Sheila Krumholz, executive director of the Center for Responsive Politics.

The huge sums have drowned out the candidates, who in many races cannot match the resources of super PACs and nonprofits, which can raise unlimited money.

Of the nearly $4 billion that the center projects will be lavished on this year’s midterms, an estimated $1.6 billion will be spent by candidates, down 7 percent from 2010.

Independent groups are “doing a lot of the dirty work in campaigns, while at the same time making it more difficult for campaigns to control the message,” said Daniel P. Tokaji, an Ohio State University professor of election law who has studied such groups’ impact on congressional races.

The net effect: Anger has been the dominant emotional appeal in this year’s Senate races, featured in two-thirds of all ads through early October, according to the Wesleyan Media Project.

The biggest imbalance this year is in Iowa, where the Senate candidates spent $13.6 million through the end of September compared with $25 million by others, according to a report by the New York University School of Law’s Brennan Center for Justice.

The disparity is also stark in House races. In the nine most competitive elections this year, House candidates have spent a total of $35 million, while independent groups and parties have poured in nearly $49 million, according to data from the nonpartisan Campaign Finance Institute.

One epicenter is in California’s Central Valley, where 23 groups have piled into the hotly contested race between first-term Democrat Ami Bera and his GOP challenger, Doug Ose.

“It’s pouring in, really from both sides,” Bera said. “It ends up muddying the waters. Elections really should be about the visions of the two candidates who are running.”

Conservatives who back campaign-finance deregulation argue that independent groups help inject new ideas into the debate, raising topics relevant to voters.

“The idea that campaigns are something solely to be hammered out between candidates and parties is very wrong,” said Bradley Smith, chairman of the Center for Competitive Politics, which supports loosening campaign finance restrictions. “There’s nothing wrong with the people saying, ‘We want this issue to be addressed.’ ”

But some of the biggest players this year — such as American Crossroads and the Senate Majority PAC — operate as shadow parties, amplifying the official campaign messages.

“It’s not like the situation 20 years ago, when independent groups were trying to change the agenda,” said Michael Malbin, executive director of the Campaign Finance Institute. “You see groups reflecting the party or candidate message, even if they are issue groups.”

The proliferation of super PACs and political nonprofits has deepened the negative tenor of campaigns. That’s in large part because the groups are prohibited from coordinating their messaging with official campaigns, making it more difficult for them to craft positive ads featuring the candidates.

The share of political ads that were positive fell in the 2010 and 2012 elections, according to Kantar Media-CMAG ad data analyzed by the Wesleyan Media Project.

“Since then, we’ve plateaued at this very high level of negativity,” said Travis Ridout, the project’s co-director.

The prominent role of independent groups in 2014 underscores how strongly they have become planted in the political firmament, less than five years after the Supreme Court opened the door to unlimited spending.

“You may not like the process and what it’s doing, but you can’t unilaterally disarm if the other side is doing it,” said Richard Hohlt, a veteran Republican consultant who has been involved in campaigns since 1972. “We’re in a spiral here that’s only going to get more and more out of control.”

Democrats, badly beaten in the race for big dollars in the past two elections, were fully engaged this year. The Senate Majority PAC, a super PAC run by top advisers to Senate Majority Leader Harry M. Reid (D-Nev.), has been one of the biggest players this cycle.

Overall, pro-GOP independent spending is still edging out pro-Democratic independent spending, according to CRP data analyzed by The Post.

The right’s advantage is probably even larger, because the tally does not include huge sums spent earlier this year by such heavyweight conservative nonprofit groups as Americans for Prosperity. That group, which is backed by billionaires Charles and David Koch and other donors on the right, is expected to invest more than $125 million in 2014.