Sheldon Adelson is also, far and away, the biggest patron of Newt Gingrich's now-surging Republican presidential bid.

Adelson and his wife, Miriam, have pumped $10 million into a political action committee backing Gingrich.

Campaign finance experts say the two $5 million contributions are among the largest known political donations in US history.

The outsized contributions are stirring some unease among the evangelical voters whom Gingrich is counting on to help him defeat Mitt Romney, his top Republican challenger.

Richard Land, head of the Ethics and Religious Liberty Commission of the Southern Baptist Convention, called the gambling cash fuelling Gingrich's bid "discomforting".

Friends say Adelson and Gingrich met when Gingrich was House of Representatives speaker and Adelson was lobbying to move the US Embassy in Israel from Tel Aviv to Jerusalem.

Gingrich backed the legislation and the two bonded over a shared hardline stance on Israel.

Gingrich this week called Adelson "very deeply concerned about the survival of Israel" and the threat of a nuclear Iran.

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 Organisation of America.

"I think that is why they became such good friends."

Gingrich has said that one of the first executive orders he would sign if elected president would move the American Embassy to Jerusalem.

Adelson's rags-to-riches story as the son of poor Ukrainian immigrants in Massachusetts is lore in the pro-Israeli circles he inhabits and where his philanthropy is legendary.

He entered the business world as a 12-year-old selling newspapers. His gambling empire stretches from Las Vegas to Macau and Singapore.

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 gift of $25 million - from Adelson.