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 Mitt Romney, people with knowledge of the contribution said Wednesday. The move leaves the Adelsons by far the most prolific campaign donors in the country.

All told, they have given at least $35 million to pro-Republican super PACs during the 2012 campaign, along with several hundred thousand dollars’ worth of $2,500 checks directly to federal candidates. That is more than twice as much money as the closest competitors for the title, the conservative Texas billionaire Harold C. Simmons and his wife, Annette, making Mr. Adelson a uniquely powerful force in the annals of presidential politics.

In the wake of court rulings and other actions that have largely deregulated the campaign finance system, wealthy donors and corporations are planning to pour hundreds of millions of dollars into this year’s presidential and Congressional elections, mostly in support of Republicans.