Casino mogul Sheldon Adelson, who helped prop up House speaker Newt Gingrich’s presidential primary campaign for months, is now using his massive wealth to help former Massachusetts governor Mitt Romney.

According to two sources familiar with the donation, Adelson is giving $10 million to the pro-Romney super PAC Restore Our Future. Under Federal Election Commission law, Restore Our Future is free to accept donations of unlimited amounts from single donors but must reveal the identity of those givers.

It’s the biggest single donation ever made to the group.

Restore Our Future, which is run by a handful of former Romney aides and confidantes, raised and spent more than $41 million during the Republican presidential primary season — money spent largely on negative television ads that went after the likes of former House Speaker Newt Gingrich and former Pennsylvania senator Rick Santorum.

During the primary, Adelson and his family gave more than $20 million to a super PAC supporting Gingrich. But even then, sources close to the billionaire said he was comfortable with Romney as the eventual nominee.

Adelson is the eighth-richest man in the world with an estimated net worth of about $25 billion dollars.

A college dropout raised in Boston by poor Jewish immigrants from Eastern Europe, Adelson pursued various business ventures before striking gold in the computer trade show industry. He used that money to revive the Sands Hotel and Casino in Las Vegas. He helped turn the Strip into a major convention destination and now owns resorts in Macao and Singapore.

Adelson started out life as a Democrat. But in the 80s and 90s, as his wealth grew and he clashed with unionized hotel workers, he became a loyal Republican. He also supports conservative politicians and causes in Israel, where he owns the newspaper Israel Hayom. He also donates millions to charity, in particular medical research.

He told the Las Vegas Sun in May that he would  was “going to give one more small donation — you might not think it's that small — to a super PAC and then if I give it will be to a C4.” (A 501c4 group does not have to disclose its donors; the GOP-backing Crossroads GPS is a likely beneficiary of Adelson’s funds.)

 “I’m against very wealthy ­people attempting to or influencing elections,” he said in a March Forbes interview. “But as long as it’s doable I’m going to do it.”

While Adelson’s financial support for Romney comes as little surprise, it does suggest that Restore Our Future, which has lain largely dormant since Romney secured the Republican presidential nomination in April, may be gearing up for another major round of ads — this time targeting President Obama.

Adelson’s donation will also stoke Democratic worries about the financial might of conservative-aligned super PACs and whether their side will be able to keep pace between now and November.

Obama is maintaining a frenetic fundraising pace. Yesterday he held his 160th fundraiser. Tomorrow night he will join Vogue editor Anna Wintour at the home of Sarah Jessica Parker and Matthew Broderick for a $40,000-a-plate dinner in New York, followed by another high-dollar gathering at the Plaza Hotel.

But Romney is quickly closing the gap. He just raised $15 million on a two-day trip to Texas; last Month he overtook Obama in fundraising for the first time.