WASHINGTON — Ten days before Donald J. Trump took office, Sheldon G. Adelson went to Trump Tower for a private meeting. Afterward, Mr. Adelson, the casino billionaire and Republican donor, called an old friend, Morton A. Klein, to report that Mr. Trump told him that moving the American Embassy from Tel Aviv to Jerusalem would be a major priority.

“He was very excited, as was I,” said Mr. Klein, the president of the Zionist Organization of America, a hard-line pro-Israel group. “This is something that’s in his heart and soul.”

The two men had to wait nearly a year, but on Wednesday, Mr. Trump stood beneath a portrait of George Washington to announce that he was formally recognizing Jerusalem as the capital of Israel and setting in motion a plan to move the embassy to the fiercely contested Holy City.

“While previous presidents have made this a major campaign promise,” he said, “they failed to deliver. Today, I am delivering.”