Billionaire Sheldon Adelson, chairman and chief executive of Las Vegas Sands Corp., and his wife, Miriam. (Anthony Kwan/Bloomberg News)

Billionaires Sheldon and Miriam Adelson shelled out $25 million in July to the main super PAC supporting Senate Republicans, bringing their support for holding the GOP majority in both chambers of Congress to at least $55 million and securing their status as the biggest donors to super PACs so far this cycle.

The $25 million from the casino magnate and his wife, a physician, marked their first donations this cycle to the Senate Leadership Fund, the super PAC aligned with Sen. Majority Leader Mitch McConnell (R-Ky.) that is working to maintain the GOP majority in the Senate this fall, according to Federal Election Commission records filed Monday evening.

In May, the couple gave $30 million to the Congressional Leadership Fund, the super PAC that backs GOP House candidates and is aligned with House Speaker Paul D. Ryan (R-Wis.). That contribution placed the Adelsons at the top of the wealthy donors who have poured money into the midterm fight. In 2016, the couple gave large sums to groups opposing Hillary Clinton’s bid for the presidency, and together they have been among the biggest political givers to conservative candidates and causes in the past three elections.

The Adelsons’ contribution made up nearly all of the Senate Leadership Fund’s fundraising haul of $26 million in July, the filings show. Among other contributors to the super PAC last month was Koch Industries, which gave $500,000, bringing the corporation’s total contributions to the super PAC this cycle to $600,000.

The couple’s infusion of cash comes as campaigns enter the final months of the midterm cycle, with Republican control of Congress hanging in the balance. Republicans are considered more likely to maintain control of the Senate, where they hold a slim majority but where Democratic incumbents face a tough defensive battle running for reelection in deep-red states that President Trump won by as much as double digits.

Groups aligned with McConnell are investing heavily in advertisements this month. In July, they announced a $16 million ad buy targeting Democratic Senate incumbents in Missouri, Indiana and North Dakota; fighting for the seat in Tennessee being vacated by Bob Corker (R), and defending one of the most vulnerable Senate Republicans this year: Dean Heller in Nevada.

The Senate Leadership Fund has raised $58 million so far this cycle, with $45 million in cash on hand, filings show. The fund and a representative for Adelson did not immediately respond to a request for comment Monday night.

On the Democrats’ side, the Senate Majority PAC (SMP) has raised $76.8 million so far and has $23.9 million in its coffers, filings show.

California physician Karla Jurvetson, a major contributor to Democratic groups this cycle, donated just over $1 million to the SMP in July, filings show. In May, she gave more than $5 million worth of stock to Women Vote, the super PAC affiliated with abortion rights group Emily’s List, which supports Democratic women who back abortion rights.

For Democratic efforts to regain control of the House, Euclidean Capital’s James Simons gave $1.7 million to the House Majority PAC. Greater New York Hospital Association Management Corp., a health-care advocacy group, shelled out $1 million last month to the Senate Majority PAC. The group’s donation in July brought its total contribution so far this cycle to $3 million.

Monday night’s FEC filings also showed the Republican National Committee maintained a major fundraising lead, pulling in $13.7 million in July for a total of $215.4 million for the 2018 midterm cycle, with $41.9 million in cash on hand.

The Democratic National Committee continues to trail, raising $7.2 million in July for a total of $113 million for the cycle, with $7.9 million in cash on hand and $6.7 million in debt, filings show.

Correction: This article previously misidentified the recipient of Greater New York Hospital Association Management Corp.'s donation.