Groups in the Israel lobby exert significant leverage over the executive branch. Influence over the executive branch derives in part from the impact pro-Israel voters and donors have on presidential elections.
Pro-Israel donors are a small part of the population. But they make large campaign donations to candidates from both parties. Hamilton Jordan was presidential adviser and former White House Chief of Staff. He wrote in a confidential memo to President Jimmy Carter: "Wherever there is major political fundraising in this country, you will find American Jews playing a significant role."
The Washington Post once estimated that Democratic presidential candidates "depend on Jewish supporters to supply as much as 60 percent of the money raised from private sources." Other estimates are lower. But pro-Israel contributions form a substantial share, between 20 and 50 percent, of the contributions made to the Democratic party and its presidential candidates.
Israel is not the only issue that inspires these contributions. But candidates who are perceived as hostile, or even indifferent, to Israel run the risk of seeing some of these funds go to their opponents.
Pro-Israel voters also have high turnout rates. They are concentrated in key states like California, Florida, Illinois, New Jersey, New York, and Pennsylvania. This increases their weight in determining who becomes president. In close races this vote can tip the balance in key states.
Jeffrey Helmreich of the Jerusalem Center for Public Affairs wrote that "American Jewish voters maintain the potential to be the decisive factor in national election results... American Jews wield power through their high concentration in key states and their tendency to behave as a swing vote."
Because this vote matters in close elections, presidential candidates go to considerable lengths to cultivate support. Candidates are especially eager to appeal to AIPAC and other organizations in the lobby. They know that the seal of approval from these prominent organizations will facilitate fund-raising and encourage higher turnout on their behalf.
Gaining and retaining that support means backing Israel down the line. This happens even though Israel strategic value has declined. It also happens even though some Israeli actions, like the continued effort to colonize the Occupied Territories, are at odds with stated US policy.
So the lobby uses American campaign money and American election mechanics to make US presidents back Israel. This happens against America own stated policy.
Source: The Israel Lobby and U.S. Foreign Policy, 2007, a book written by John Mearsheimer and Stephen Walt