A dozen major donors are responsible for $1 of every $13 contributed to federal candidates and committees since the Citizens United Supreme Court ruling that allowed for unlimited independent spending to influence elections, according to a new report from a campaign finance watchdog group.

The donors and their spouses gave $3.4 billion since the 2010 election cycle, accounting for 7.5% of the $45 billion given to all federal candidates since the 2010 election cycle, according to Issue One, a nonpartisan group that advocates for reducing the role of money in politics.

Of the biggest donors, six were aligned with Republicans and six with Democrats. Topping the list were two donors who also self-financed their 2020 Democratic presidential campaigns: former New York City Mayor Michael Bloomberg and former hedge fund manager Tom Steyer. In total, Bloomberg gave $1.4 billion in the period and Steyer $653 million.

Bloomberg is the founder and majority owner of Bloomberg LP, the parent company of Bloomberg News.

Jason Schechter, a spokesman for Bloomberg, declined to comment. A spokesperson for Steyer didn’t immediately return a message seeking comment.

In January 2010, the court ruled that independent spending on elections by corporations was protected by the First Amendment, opening the door to unlimited spending by super-political action committees and touching off an arms race of fundraising by groups seeking to influence elections.

“Our government can’t be responsive to all Americans if our elected officials are beholden to the elite donor class,” said Nick Penniman, the founder and chief executive officer of Issue One.

Other top Democratic donors included S. Donald Sussman, founder of the hedge fund Paloma Partners who gave $98 million, and Renaissance Technologies Founder James Simons and his wife Marilyn Simons who donated $96 million.

The late Sheldon Adelson and his wife, Miriam Adelson, were the top Republican donors, giving $523 million, followed by Richard Uihlein and Elizabeth Uihlein, owners of closely held Uline Inc., and Citadel’s Ken Griffin.

Donations to federal candidates, parties and other committees like super PACs were included in Issue One’s analysis. While there are restraints on donations to campaigns and parties, donations to super PACs are unlimited.

Big political donors tend to live in just a handful of wealthy zip codes. The top areas for campaign contributions are home to 2.5 million people, less than 1% of the U.S. population, but they accounted for 20% of the $45 million donated to federal candidates and committees.

This article was provided by Bloomberg News.