He’s an ardent supporter of Israel. A mega-billionaire casino mogul whose Las Vegas Sands Corp. is under federal investigation. And the self-proclaimed “richest Jew in the world.”

Sheldon Adelson is also, far and away, the biggest patron of Newt Gingrich’s surging Republican presidential bid. Adelson and his wife, Miriam, have pumped $10 million into a political action committee backing Gingrich that is run by the former House speaker’s onetime aides. Campaign finance experts say the two $5 million contributions are among the largest known political donations in U.S. history.

No other candidate in the race for president appears to be relying so heavily on the fortune of a single donor. It’s been made possible by last year’s Supreme Court rulings — known as Citizens United — that recast the political landscape by stripping away restrictions on contributions and how outside groups can spend their money.

Sheldon Adelson is Citizens United come to life.

“The bottom line is that it creates that potential for one person to have far more influence than any one person should have,” said Fred Wertheimer, president of the campaign finance watchdog group Democracy 21.

When any candidate is beholden to a single donor for so much money, Wertheimer said, “it opens the door to corruption and influence peddling.” Wertheimer said the infusion of cash would raise questions about any decision Gingrich would make that touches on gambling, for example. And similar questions could be raised about Gingrich’s Mideast policies.

Indeed, without recent disclosures by news organizations, voters would not have even known about the large contributions until campaign filings due Feb. 20. That would be long after a number of key primaries.

The outsized contributions are stirring some unease among the evangelical voters whom Gingrich is counting on to help him defeat Mitt Romney. Richard Land, head of the Ethics and Religious Liberty Commission of the Southern Baptist Convention, called the gambling cash fueling Gingrich’s bid “discomforting.”

Land said Gingrich should make clear what his views are on legalized gambling.

Gingrich spokesman R.C. Hammond said the candidate believes it is a states’ rights issue and does not gamble.

Friends say Adelson and Gingrich met when Gingrich was House speaker and Adelson was lobbying to move the U.S. Embassy in Israel from Tel Aviv to Jerusalem. Gingrich backed the legislation and the two bonded over a shared hardline stance on Israel.

In Cocoa, Fla., Gingrich on Wednesday called Adelson “very deeply concerned about the survival of Israel” and the threat of a nuclear Iran. Asked if he had promised Adelson anything, Gingrich replied that he pledged “that I would seek to defend the United States and United States allies.”

Those who have followed Gingrich’s career say he has long staked out a tough stance on Israel that predated his friendship with Adelson.

Gingrich “has been one of the few politicians who has had the courage to tell the truth about Israel,” said Morton Klein, head of the Zionist Organization of America. “I think that is why they became such good friends.”

In December, Gingrich proclaimed the Palestinians “an invented people.” Israel’s Haaretz daily reported later that month that Adelson approved of the remarks. And Gingrich has said that one of the first executive orders he would sign if elected president would move the American Embassy to Jerusalem.

Through a spokesman, Adelson declined an interview request from The Associated Press.

His rags-to-riches story as the son of poor Ukrainian immigrants in Dorchester, Mass., is well-known lore in the pro-Israeli circles he inhabits and where his philanthropy is legendary.

Adelson entered the business world as a 12-year-old selling newspapers. He began to make his fortune when he founded Comdex, a trade show that became a staple for the computer industry. He then moved into the casino industry. His gambling empire stretches from Las Vegas to Macau and Singapore and includes the Venetian and Palazzo casinos in Las Vegas.

The FBI and the Securities and Exchange Commission are investigating Adelson for possible violation of the Foreign Corrupt Practices Act, according to a filing with the SEC. The company denies any wrongdoing and says the investigation stems from the allegation of a disgruntled employee.

The son of a cab driver, Adelson now ranks as the eighth wealthiest person in America, according to Forbes Magazine, which places his net worth at $21.5 billion.

Last year, Yad Vashem, the Holocaust memorial in Israel, said it received its largest private donation ever — a $25 million gift — from Adelson. Since 2007, he has donated more than $100 million to Birthright Israel, a group that sends young adult Jews from the United States and other countries on 10-day trips to Israel.

Adelson is an outspoken supporter of Israeli Prime Minister Benjamin Netanyahu and owns a widely read, right-wing Israeli newspaper, Israel Hayom, which is distributed at no cost throughout Israel and is supportive of Netanyahu.

The hefty donations to Gingrich’s presidential bid aren’t the first checks he’s written to help the former Georgia congressman. He ponied up more than $7 million to help get Gingrich’s conservative political group American Solutions for Winning the Future off the ground.

The first $5 million donation from Adelson came at a critical juncture for Gingrich as he entered South Carolina, stung by a humbling fifth-place finish in New Hampshire’s Republican primary. The Adelson money to Winning Our Future, a pro-Gingrich PAC led by formerGingrich aide Rick Tyler, helped finance a 28-minute movie bashing Mitt Romney’s tenure at the helm of the private equity firm Bain Capital.

Gingrich was able to leverage the support into a double-digit win in South Carolina over Romney.

Presumably pleased with his investment, Adelson doubled down in Florida, where the next Republican contest will take place Jan. 31. This week, Adelson’s wife chipped in another $5 million. The money is quickly going right back out the door.

Tyler told the AP that Winning Our Future had made a $6 million ad buy in Florida. A spot is planned to take aim at Romney’s health care plan as governor of Massachusetts and its connection to President Barack Obama’s health care overhaul, Tyler said.

Feature photo | Republican mega-donor Sheldon Adelson listens as New Jersey Gov. Chris Christie speaks during the Republican Jewish Coalition, March 29, 2014, in Las Vegas. Julie Jacobson | AP