Billionaire casino magnate Sheldon Adelson has given $30 million to a super PAC pushing to protect the House majority, and is leaving the door open to putting millions more toward the Republican midterm effort this year, a source with direct knowledge told CNBC.

The cash influx to the GOP-linked Congressional Leadership Fund gives Republicans a massive boost in resources as they try to fight off Democrats set on flipping the House in November. Democrats, emboldened by party enthusiasm and opposition to congressional GOP policies, aim to flip 23 GOP-held seats to take a House majority.

Adelson's donation was first reported by Politico.

Adelson, the Las Vegas Sands chief executive and a major donor to GOP causes, gave the organization $10 million in the later stages of the 2016 election cycle. This year, he got involved earlier as Republicans try to hold off energized Democrats.

Last week, a group including House Speaker Paul Ryan met with Adelson and his wife, Miriam, in Las Vegas, Politico reported and CNBC confirmed. Those gathered sold the casino executive on the importance of the 2018 midterms, the source said.

Ryan, who cannot legally push for donations that large, left the room before a deal on the massive contribution was reached. Adelson likely will not give more money to the Congressional Leadership Fund, specifically, during this election cycle, the source told CNBC.

Spokespeople for Adelson and Ryan's political team declined to comment. A Congressional Leadership Fund spokesman did not immediately respond to a request to comment.

Adelson, 84, is likely pleased with the policies pursued by the Republican-controlled Congress and White House. In April, Las Vegas Sands reported an income tax benefit of $670 million due to the GOP tax law passed in December. The overhaul chopped tax rates for corporations and reduced income taxes for most individuals.

Adelson, who is staunchly pro-Israel, has supported the Trump administration's decision to move the U.S. embassy to Jerusalem. He has also opposed the Iran nuclear deal. Trump said this week that the U.S. was withdrawing from the pact.

The casino mogul's massive donation is likely to prompt more attacks from Democrats on the influence of wealthy individuals on elections. Responding to Politico's report, Sen. Bernie Sanders, I-Vt., tweeted that "democracy means one person, one vote. It does not mean billionaires like Sheldon Adelson buying elections."

Sanders tweet

In a statement, Democratic Congressional Campaign Committee spokesman Tyler Law said "there is simply no price on the amount of grassroots energy surging behind House Democrats this cycle." The House Democrats' campaign arm has recognized that Republican outside groups would likely outspend organizations on the Democratic side, "and we take nothing for granted," he added.