Evan Vucci, Associated Press
Republican presidential candidate, former House Speaker Newt Gingrich speaks on the floor of the Oklahoma State Legislature in Oklahoma City, Okla., Tuesday, Feb. 21, 2012.

WASHINGTON — For every presidential candidate, there's now at least one $1 million donor. Millionaire and billionaire executives have unlocked their personal bank vaults to write seven-figure checks to support the campaigns of President Barack Obama and Republicans Mitt Romney, Rick Santorum, Newt Gingrich and Ron Paul.

More than half the $60 million collected so far by super political committees in the race came from just 24 wealthy Americans, according to a review of campaign financial reports by The Associated Press. In some cases, the contributions of $1 million or more represent most of the money that super PACs have collected from all their contributors.

These outsized donations — more than 40 times as much as ordinary Americans can give in cash directly to a politician — are allowed under the landmark 2010 Supreme Court ruling, known as the Citizens United case.

The ruling in the Citizens United case made it possible for so-called "super" political action committees to raise and spend unlimited sums to support political campaigns. The super PACs must legally remain independent from the candidates they support, but many are staffed with former campaign aides who have intimate knowledge of the campaigns' strategy.

"It's just so much easier for these people to make large contributions and play a much more prominent role than we've tended to see," said Brendan Glavin, a researcher with the Campaign Finance Institute, a Washington-based nonpartisan think tank.

Here's a list of each presidential candidate's wealthiest supporters:

—Even among seven-figure donors, nobody approaches the $11 million that Las Vegas casino billionaire Sheldon Adelson and his family have given to the group supporting former House Speaker Gingrich, which has just $13.1 million in total contributions. Adelson gave $5 million; his wife, Miriam, another $5 million. The rest came in smaller amounts from Adelson's daughters, Sivan Ochshorn and Yasmin Lukatz, and Lukatz's husband, Oren. Gingrich has said Adelson and his family support his strong pro-Israel statements. Adelson has important business interests in China, and his casino is under federal investigation by the Justice Department and a civil probe by the Securities and Exchange Commission for possible violations of the Foreign Corrupt Practices Act. The company denies wrongdoing.

—The cofounder of PayPal and an early investor in Facebook, Peter Thiel, has given $2.6 million to the group supporting Texas Rep. Paul, the Endorse Liberty super PAC. Nearly 70 percent of the group's money comes from Thiel, the Silicon Valley investor. An ardent supporter of Libertarian causes, Thiel has donated to gay rights and religious organizations and also helped fund the Committee to Protect Journalists. His Facebook investment alone is now reportedly worth $1.3 billion.

—The chief executive at Dreamworks Animation, Jeffrey Katzenberg, has given $2 million to the group supporting Obama's re-election, Priorities USA Action, and accounts for nearly half the group's $4.5 million total. Katzenberg has hosted fundraisers for Obama and has been a long-time funder of liberal and Democratic Party causes.

—Hotel magnates and brothers Bill and Richard Marriott have given $1.5 million to the group supporting former Massachusetts Gov. Romney, Restore Our Future. Bill Marriott heads Marriott International, while Richard Marriott heads offshoot Host Hotels & Resorts. Romney, whose first name is Willard, was named after the Marriotts' father, who was close friends with his own father, George Romney. Mitt Romney also served for years on Marriott's board of directors.

—Julian Robertson, the head of Tiger Management Corp., a major New York-based hedge fund, gave Restore Our Future $1.3 million to help Romney. Robertson's son is also a Romney donor, and both have co-chaired Romney fundraising events.

—Other rich executives who gave at least $1 million to help Romney's campaign include Paul Edgerly, a managing director at Bain Capital, and his wife Sandra; Edward Conard, a former Bain executive; Mormon businessman Frank VanderSloot; Paul Singer, founder and CEO of Elliott Management Corp., a top New York-based hedge fund; John Paulson, head of Paulson and Company, another major hedge fund; Bob Perry, a Houston real estate developer; Tulsa businessman Francis Rooney; Robert Mercer, co-chief of Renaissance Technologies, a New York hedge fund; and William Koch, head of Oxbow Carbon, LLC, a fossil fuel processor and mining firm.

—The group supporting former Pennsylvania Sen. Santorum — the Red, White and Blue Fund — collected nearly all its $2.8 million so far from just two major, $1 million contributions. Foster S. Friess, a mutual fund entrepreneur and conservative advocate, is a major funder behind the Daily Caller, a conservative web news aggregator, and has his own website advocating free enterprise and warning about militant Islam. Friess opposed the Obama administration's health care program and gave $100,000 to statewide Republican races and causes in 2008.

—The pro-Santorum group's other $1 million contribution came from William J. Dore, president of a Louisiana energy and real estate firm. Dore backed New Orleans Mayor Mitch Landrieu and other state Democrats, but he has also funded Republican politicians, as well as charities in Louisiana and Texas.

—The pro-Gingrich super PAC Winning Our Future also gained $1 million from Texas billionaire Harold Simmons, a longtime Republican donor. Simmons also donated $12 million — both personally and through his firm, Contran — to American Crossroads, the Republican-leaning super PAC co-founded by longtime Republican strategist Karl Rove.