The list of the 11 biggest super-PAC donors comprises five Republicans, five Democrats and former New York mayor Michael Bloomberg, who previously had declared himself a political independent and this month registered as a Democrat.

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The intense concentration of money shows how a tiny group of super-rich individuals has embraced these political groups, which have emerged as indispensable allies of candidates and political parties since the Supreme Court’s landmark Citizens United decision in 2010. That ruling helped give rise to super PACs, which are allowed to raise and spend unlimited amounts of money on political activity.

The massive sums coming from just a handful of donors illustrate how candidates and parties are now dependent on billionaires to support their efforts — and demonstrate the successful circumvention of efforts to curtail big money in politics that followed the Watergate and 1996 Democratic National Committee fundraising scandals.

“The big donor is not just a donor who gives to politicians and parties,” said Robert E. Mutch, a campaign-finance historian. “The big donor has become a political actor in his own right.”

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Many of the top 11 super-PAC givers since 2010 are now well-known protagonists in U.S. politics. Billionaire hedge-fund founders George Soros (No. 10) and Tom Steyer (No. 2) were among the mrore than a dozen critics of President Trump targeted with explosive devices this week. And Bloomberg (No. 3) and Steyer are viewed as positioning themselves for possible presidential runs.

Big money has continued to balloon in this year’s midterms, even amid a surge of small-dollar donations. In some races, huge super-PAC contributions are bolstering candidates who are touting their grass-roots support. For example, Katie Hill, a first-time Democratic candidate in California’s 25th Congressional District and one of the most successful fundraisers this election, is also being buoyed by $4.5 million in ads in the final stretch of the campaign, thanks to Independence USA, Bloomberg’s super PAC.

The largest super-PAC contributors are casino mogul Sheldon Adelson and physician Miriam Adelson, the married couple who have given $287 million to conservative super PACs, records show.

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In the 2016 elections, the Adelsons gave nearly $78 million, including $20 million to bolster then-GOP nominee Donald Trump, who as president backed an action the couple had long sought — moving the U.S. Embassy in Israel from Tel Aviv to Jerusalem.

This cycle, the Adelsons have given $112 million to super PACs, largely to groups working to help the GOP retain control in Congress.

A representative for the Adelsons did not respond to a request for comment.

In second place behind the Adelsons is Steyer, who has given $213.8 million. He is followed by Bloomberg ($123.4 million), Democratic media executive Fred Eychaner ($68 million), Democratic hedge-fund executive Donald Sussman ($62.9 million), Republican shipping-supplies magnate Richard Uihlein ($59.9 million), Democratic hedge-fund founder James Simons and his wife, Marilyn ($57.9 million), Republican hedge-fund executive Paul Singer ($41.9 million), Republican hedge-fund executive Robert Mercer ($40.9 million), Soros ($39.4 million) and Republican backer and TD Ameritrade founder J. Joe Ricketts ($38.4 million).

Their total giving could be substantially more. While donors to super PACs are disclosed, public filings do not reflect contributions to politically active nonprofit groups that are not required to reveal their donors.

Super PACs are prohibited from coordinating with campaigns and party committees. But they often act on parallel tracks, and with each election these groups have found ways to work even more closely with the candidates they are backing. By 2016, super PACs of presidential contenders were conducting their own advertising campaigns, research, polling and voter turnout activities in support of their favored candidates.

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“Super PACs have stepped in to take over a lot of the campaign functions that we used to expect from candidates,” said David Magleby, a Brigham Young University political-science professor who studies super PACs. “They’re playing a very large role in virtually every part of the American democracy.”

Critics say super PACs allow wealthy donors to have a disproportionate influence on politics.

“One thing is clear — super PACs are a rich man’s game,” said Meredith McGehee, executive director of Issue One, a nonpartisan group that seeks to lessen the influence of big money on politics. “They’re trying to exert influence because they believe whatever particular party they are supporting is either in ideological agreement with the donor or because they have business or other interests that they believe those particular sets of candidates will be sympathetic to.”

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David Keating, an opponent of campaign finance restrictions who helped bring a case, SpeechNow.org v. FEC, that led to the creation of super PACs, said the groups help political donors exercise their right to free speech.

“Rich people have First Amendment rights, too,” Keating said.

Steven Law, president of the Senate Leadership Fund, which is aligned with Senate Majority Leader Mitch McConnell (R-Ky.) and has received big checks from the Adelsons, said he could not speculate on what motivates individual donors.

But Law said that the recent Supreme Court confirmation battle over Brett M. Kavanaugh “highlighted for our donors the importance of the Supreme Court and the lengths to which Democrats would go to block all future nominations if they got control of the Senate.”

After initially shunning super PACs when they came on the scene in 2010, Democrats have since embraced them with gusto.

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In an interview, Steyer said he agrees with the critics of Citizens United and said the rise of big money in politics is “corrosive and corrupting and has led us to a terrible place.”

He said he views his super-PAC contributions — more than $50 million this cycle, largely to two super PACs he started — as a way of fighting fire with fire.

“If you look and see where the overwhelming bulk of our money has gone, it’s to try to organize and engage people in the political process around the country,” Steyer said.

Bloomberg — who has spent more than $61 million supporting Democrats this cycle, a figure that could soar to $100 million — is working for “a change of direction in Washington,” according to a spokesman.

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Many of the top super-PAC donors on the left have plowed their money into the main party-aligned super PACs — Priorities USA Action, Senate Majority PAC and House Majority PAC.

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Jeb Fain, spokesman for House Majority PAC, said the group is “proud to have strong and continuing support” from its donors. Their contributions have helped the group launch a barrage of advertisements and “fill spending gaps by working with our allies to ensure total Democratic spending matches or exceeds Republicans in the critical races we need to win in order to flip the House,” he said.

Democratic strategists said that accepting large donations is necessary to combat the money on the right — particularly the resources of the Koch network, a well-established constellation of organizations and donors, notably billionaire industrialists Charles and David Koch, that is unmatched by any network on the left. (David Koch retired from business and political life in June amid declining health.)

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“Since 2010, Democrats have scraped and clawed to compete with the Koch brothers, Sheldon Adelson and Richard Uihleins of the world that have contributed hundreds of millions of dollars to Republican election efforts,” said J.B. Poersch, Senate Majority PAC’s president. “Donors give to Senate Majority PAC because they know we give them the best opportunity to cancel out the Koch brothers and elect Democrats to the Senate.”