Kin Cheung/Associated Press

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Even in a political season marked by unprecedented levels of political spending, Sheldon Adelson stands alone.

In recent days, Mr. Adelson, a billionaire casino owner, and his wife, Dr. Miriam Adelson, gave $10 million to Restore Our Future, a “super PAC” backing the Republican presidential candidate Mitt Romney, people with knowledge of the contribution said on Wednesday, leaving the Adelsons by far the most prolific campaign donors in the country.

All told, the Adelsons have now given at least $35 million to super PACs during the 2012 campaign, not including several hundred thousand dollars worth of $2,500 contributions directly to federal candidates. That is more than twice as much as his closest competitor for the title, the billionaire industrialist Harold C. Simmons and his wife, Annette, making Mr. Adelson a uniquely powerful force in the annals of presidential politics.

With a net worth of roughly $25 billion, Mr. Adelson could afford to give far more, and seems willing to do so: In an interview with Forbes magazine this year, he suggested he would consider spending as much as $100 million on the 2012 elections.

Until recently, most of the Adelsons’ contributions went to Winning Our Future, a super PAC that provided critical support to Newt Gingrich, Mr. Romney’s rival during the Republican primaries. Winning Our Future spent millions of dollars pounding Mr. Romney with advertisements this winter, injecting into the race some of the toughest and most sustained attacks on his record at the private equity firm Bain Capital. And Mr. Adelson has publicly expressed doubts about Mr. Romney, describing him as timid compared with Mr. Gingrich.

But although Mr. Romney is barred from asking Mr. Adelson to contribute large checks to the super PAC, he has been courting the billionaire since January, around the time Mr. Adelson and his wife began pouring millions of dollars into Winning Our Future. At the time, Mr. Adelson relayed assurances that he would support Mr. Romney’s candidacy even more generously should he become the nominee.

After Mr. Gingrich dropped out in early May, Mr. Romney and Mr. Adelson met for lunch later in the month at Mr. Adelson’s Las Vegas hotel, The Venetian. Last week, Politico reported that Mr. Adelson was considering a contribution of $1 million to $10 million to Restore Our Future, the pro-Romney super PAC.

Mr. Adelson is close to many of Mr. Romney’s other supporters, particularly wealthy Jewish donors, with whom he has served on the board of the Republican Jewish Coalition. And he has ties to Restore Our Future: the group’s political director, Carl Forti, helped run Freedom’s Watch, an issue advocacy group that spent $17 million during the 2008 election.