Through a spokesman, the Adelsons declined a request to be interviewed.
The Adelsons’ growing influence comes as other Republican megadonors — Charles and David Koch, the billionaire industrialists; Robert Mercer and his daughter, Rebekah, two of Mr. Trump’s most influential supporters; Foster Friess, a major benefactor to conservative causes; and Dick and Liz Uihlein, the Midwestern couple who have written big checks to anti-establishment candidates — have scaled back their spending or placed bad bets on losing campaigns.
So far this year the Adelsons have steered their money to two groups with proven track records of defending Republican seats — the Congressional Leadership Fund, the super PAC allied with Speaker Paul D. Ryan, and the Senate Leadership Fund, which has close ties to Senator Mitch McConnell, the Republican leader. Though they are known to probe prospective recipients about their plans up front, they do not tend to meddle in strategy after the fact or demand that their money be steered toward certain states or races, people who have worked with them said.
Their $55 million commitment dwarfs their contribution during the last midterms in 2014, when Republicans were in a much stronger position politically and the couple was more focused on projects in Nevada. That year, they had donated only $382,000 as of Aug. 31 to federal campaigns, and gave only $5.5 million overall. In 2016, a presidential year, they had donated $46.5 million by this point.
They have yet to give to Mr. Trump’s re-election efforts, however, reasoning that for now their money is better spent on maintaining Republican control of Congress.
Representatives from America First Policies, the super PAC that is acting as the main vehicle for Mr. Trump’s 2020 effort, recently visited Mr. Adelson in Las Vegas to make a pitch for financial support, according to several people who had been briefed on the meeting. One of these people said Mr. Adelson told associates afterward that he was not convinced of their strategic plan, saying it was too vague and unformed, but has not ruled out donating in the future.
The relationship between Mr. Trump and Mr. Adelson has come a long way.
In the fall of 2015, Mr. Trump attacked Mr. Adelson on Twitter after rumors swirled that the casino mogul might support a rival for the Republican nomination, Senator Marco Rubio of Florida. “He feels he can mold him into his perfect little puppet. I agree!” Mr. Trump vented.