Several years ago, I had dinner at Galileo, a Washington restaurant, with Steven Rosen, who was then the director of foreign-policy issues at the American Israel Public Affairs Committee. The group, which is better known by its acronym, aipac, lobbies for Israel’s financial and physical security. Like many lobbyists, Rosen cultivated reporters, hoping to influence their writing while keeping his name out of print. He is a voluble man, and liked to demonstrate his erudition and dispense aphorisms. One that he often repeated could serve as the credo of K Street, the Rodeo Drive of Washington’s influence industry: “A lobby is like a night flower: it thrives in the dark and dies in the sun.”

Lobbyists tend to believe that legislators are susceptible to persuasion in ways that executive-branch bureaucrats are not, and before Rosen came to aipac, in 1982 (he had been at the rand Corporation, the defense-oriented think tank), the group focussed mainly on Congress. But Rosen arrived brandishing a new idea: that the organization could influence the outcome of policy disputes within the executive branch—in particular, the Pentagon, the State Department, and the National Security Council.

Rosen began to court officials. He traded in gossip and speculation, and his reports to aipac’s leaders helped them track currents in Middle East policymaking before those currents coalesced into executive orders. Rosen also used his contacts to carry aipac’s agenda to the White House. An early success came in 1983, when he helped lobby for a strategic coöperation agreement between Israel and the United States, which was signed over the objections of Caspar Weinberger, the Secretary of Defense, and which led to a new level of intelligence sharing and military sales.

aipac is a leviathan among lobbies, as influential in its sphere as the National Rifle Association and the American Association of Retired Persons are in theirs, although it is, by comparison, much smaller. (aipac has about a hundred thousand members, the N.R.A. more than four million.) President Bush, speaking at the annual aipac conference in May of 2004, said, “You’ve always understood and warned against the evil ambition of terrorism and their networks. In a dangerous new century, your work is more vital than ever.” aipac is unique in the top tier of lobbies because its concerns are the economic health and security of a foreign nation, and because its members are drawn almost entirely from a single ethnic group.

aipac’s professional staff—it employs about a hundred people at its headquarters, two blocks from the Capitol—analyzes congressional voting records and shares the results with its members, who can then contribute money to candidates directly or to a network of proIsrael political-action committees. The Center for Responsive Politics, a public-policy group, estimates that between 1990 and 2004 these pacs gave candidates and parties more than twenty million dollars.

Robert H. Asher, a former aipac president, told me that the pacs are usually given euphemistic names. “I started a pac called Citizens Concerned for the National Interest,” he said. Asher, who is from Chicago, is a retired manufacturer of lamps and shades, and a member of the so-called Gang of Four—former presidents of aipac, who steered the group’s policies for more than two decades. (The three others are Larry Weinberg, a California real-estate developer and a former owner of the Portland Trail Blazers; Edward Levy, a construction-materials executive from Detroit; and Mayer “Bubba” Mitchell, a retired builder based in Mobile, Alabama.)

aipac, Asher explained, is loyal to its friends and merciless to its enemies. In 1982, Asher led a campaign to defeat Paul Findley, a Republican congressman from Springfield, Illinois, who once referred to himself as “Yasir Arafat’s best friend in Congress,” and who later compared Arafat to Gandhi and Martin Luther King, Jr.

“There was a real desire to help Findley out of Congress,” Asher said. He identified an obscure Democratic lawyer in Springfield, Richard Durbin, as someone who could defeat Findley. “We met at my apartment in Chicago, and I recruited him to run for Congress,” he recalled. “I probed his views and I explained things that I had learned mostly from aipac. I wanted to make sure we were supporting someone who was not only against Paul Findley but also a friend of Israel.”

Asher went on, “He beat Findley with a lot of help from Jews, in-state and out-of-state. Now, how did the Jewish money find him? I travelled around the country talking about how we had the opportunity to defeat someone unfriendly to Israel. And the gates opened.” Durbin, who went on to win a Senate seat, is now the Democratic whip. He is a fierce critic of Bush’s Iraq policy but, like aipac, generally supports the Administration’s approach to the Israeli-Palestinian conflict. Durbin says that he considers Asher to be his “most loyal friend in the Jewish community.”

Mayer Mitchell led a similar campaign, three years ago, to defeat Earl Hilliard, an Alabama congressman who was a critic of Israel. Mitchell helped direct support to a young Harvard Law School graduate named Artur Davis, who challenged Hilliard in the Democratic primary, and he solicited donations from aipac supporters across America. Davis won the primary, and the seat. “I asked Bubba how he felt after Davis won,” Asher said, “and he said, ‘Just like you did when Durbin got elected.’ ” Mitchell declined to comment.

aipac’s leaders can be immoderately frank about the group’s influence. At dinner that night with Steven Rosen, I mentioned a controversy that had enveloped aipac in 1992. David Steiner, a New Jersey real-estate developer who was then serving as aipac’s president, was caught on tape boasting that he had “cut a deal” with the Administration of George H. W. Bush to provide more aid to Israel. Steiner also said that he was “negotiating” with the incoming Clinton Administration over the appointment of a pro-Israel Secretary of State. “We have a dozen people in his”—Clinton’s—“headquarters . . . and they are all going to get big jobs,” Steiner said. Soon after the tape’s existence was disclosed, Steiner resigned his post. I asked Rosen if aipac suffered a loss of influence after the Steiner affair. A half smile appeared on his face, and he pushed a napkin across the table. “You see this napkin?” he said. “In twenty-four hours, we could have the signatures of seventy senators on this napkin.”

Rosen was influential from the start. He was originally recruited for the job by Larry Weinberg, one of the Gang of Four, and he helped choose the group’s leaders, including the current executive director, Howard Kohr, a Republican who began his aipac career as Rosen’s deputy. Rosen, who can be argumentative and impolitic, was never a candidate for the top post. “He’s a bit of a kochleffl”—the Yiddish term for a pot-stirrer, or meddler—Martin Indyk, who also served as Rosen’s deputy, and who went on to become President Clinton’s Ambassador to Israel, says. Rosen has had an unusually eventful private life, marrying and divorcing six times (he is living again with his first wife), and he has a well-developed sense of paranoia. When we met, he would sometimes lower his voice, even when he was preparing to deliver an anodyne pronouncement. “Hostile ears are always listening,” he was fond of saying.