Las Vegas casino magnate Sheldon Adelson sits with his wife Miriam as they await the start of the first debate between Republican U.S. presidential nominee Donald Trump Democratic and U.S. presidential nominee Hillary Clinton at Hofstra University in Hempstead, New York, U.S., September 26, 2016.

Republican megadonors Sheldon and Miriam Adelson combined to give $25 million last month to the Senate Leadership Fund, a super PAC dedicated to protecting the GOP majority in the U.S. Senate.

The Adelsons each gave $12.5 million. Their combined contribution made up nearly all of the $26 million haul that the PAC raised in July, according to new filings with the Federal Election Commission.

The donations mark the latest bid by the casino magnate and his wife, a physician, to support Republican candidates during this fall's midterm elections.

In May, the Adelsons dished out $30 million to the Congressional Leadership Fund, a super PAC which aims to keep Republicans in control of the House.

Sheldon Adelson, CEO of casino behemoth Las Vegas Sands, separately has given $371,500 to Republican joint fundraising committee Protect the House. A leading advocate for Israeli interests in the Middle East, Adelson also has given thousands of dollars to GOP candidates who have largely spoken out against the Iran deal.

While Democrats are slightly favored to flip the House this fall, Republicans are widely expected to hold on to the Senate. The GOP only has to defend nine Senate seats in the midterms, compared with the 26 seats up for grabs that are held by Democrats and independents who caucus with Democrats.

Other prominent donors to the Senate Leadership Fund last month include the billionaire Koch brothers' Koch Industries, which gave $500,000; coal miner Murray Energy, which gave $15,000; and former Treasury Secretary and Goldman Sachs CEO Henry Paulson, who gave $30,000.