In a letter to prominent Jewish leaders, Hillary Clinton expressed her “alarm” over calls to boycott Israeli products to push Israel to improve its dealings with the Palestinians. Clinton has largely avoided discussion of foreign policy in her early months on the campaign trail. But in the July 2 letter, whose recipients included some of the biggest donors to her campaigns and a “super PAC” supporting her, she said the Boycott, Sanctions and Divestment, or B.D.S., campaign against Israel was “the latest effort to single out Israel on the world stage.” The letter was provided to news organizations by a public relations firm working for Haim Saban, the Hollywood mogul who has contributed at least $2 million to a “super PAC” backing Clinton, also spoke at an anti-B.D.S. conference in Las Vegas in June hosted by Sheldon Adelson, a casino billionaire and leading Republican donor.