Ari Berman writes in The Nation:

AIPAC is the leading player in what is sometimes referred to as “The Israel Lobby” — a coalition that includes major Jewish groups, neoconservative intellectuals and Christian Zionists. With its impressive contacts among Hill staffers, influential grassroots supporters and deep connections to wealthy donors, AIPAC is the lobby’s key emissary to Congress. But in many ways, AIPAC has become greater than just another lobby; its work has made unconditional support for Israel an accepted cost of doing business inside the halls of Congress. AIPAC’s interest, Israel’s interest and America’s interest are today perceived by most elected leaders to be one and the same. Christian conservatives increasingly aligned with AIPAC demand unwavering support for Israel from their Republican leaders. (In mid-July, 3,000-plus evangelicals came to town for the first annual “Christian United for Israel” summit.) And Democrats are equally concerned about alienating Jewish voters and Jewish donors — long a cornerstone of their party. Some in Congress are deeply uncomfortable with AIPAC’s militant worldview and heavyhanded tactics, but most dare not say so publicly.
“The Bush Administration is bad enough in tolerating measures they would not accept anywhere else but Israel,” says Henry Siegman, the former head of the American Jewish Congress and a Middle East expert at the Council on Foreign Relations. “But the Congress, if anything, is urging the Administration on and criticizing them even at their most accommodating. When it comes to the Israeli-Arab conflict, the terms of debate are so influenced by organized Jewish groups, like AIPAC, that to be critical of Israel is to deny oneself the ability to succeed in American politics.”

Full story.
[Update] Oh, I also meant to post this WaPo article about AIPAC and the America-Israel relationship two weeks ago…