WASHINGTON — The return on investment for many of the Republican Party’s biggest political patrons has been less than impressive this year. But not for Sheldon Adelson.

Mr. Adelson, the billionaire casino magnate, and his wife, Miriam, a physician, have emerged as the biggest and potentially most influential contributors to Republicans in the midterm season. Despite initially harboring qualms about President Trump’s leadership, the Adelsons have found much to like in a Republican-controlled government that has aligned with their most cherished priorities: unflinchingly pro-Israel, unaccommodating to Middle Eastern adversaries and dedicated to deregulation and lower taxes.

Mr. Adelson in particular enjoys a direct line to the president. In private in-person meetings and phone conversations, which occur between the two men about once a month, he has used his access to push the president to move the United States embassy in Israel to Jerusalem and, more recently, cut aid to the Palestinians, according to people familiar with their discussions, who spoke anonymously to discuss private matters. Mr. Trump has done both, triggering a backlash from some American allies.

Republican control of the House and the Senate is so vital to maintaining these policies, the Adelsons believe, that they have given $55 million in the last few months to groups dedicated to making sure it stays that way. That makes them not only the largest donors to national Republican electoral efforts in this election cycle, but the biggest spenders on federal elections in all of American politics, according to publicly available campaign finance data.