A couple of weeks ago, in the wake of Benjamin Netanyahu's speech to a joint meeting of Congress, Andrew Sullivan asked the following question: "(W)ithout the power of (the pro-Israel) lobby, how do you make any empirical sense of last week's events at all?" What he meant was, how can you possibly explain the reception Netanyahu received in Congress (something like 30 standing ovations), and how can you explain the decision by the Senate majority leader, Harry Reid, to contradict the President on the matter of 1967 borders? Andrew now interprets Israel's power in Washington in the manner of Stephen Walt and John Mearsheimer, whose anti-Israel polemic, "The Israel Lobby," blames American Jewish supporters of Israel for most of the bad things that have happened to America abroad over the past decade. Their argument is simple: Without "the Lobby," Israel would be friendless in Washington.

This always struck me as wrong, not because AIPAC isn't powerful, but because Walt and Mearsheimer (and Andrew) don't seem to understand what makes a powerful lobby group powerful. The most powerful lobbies, over time, are those that lobby for causes that are already popular among the American people.
 
Walter Russell Mead understands this phenomenon, and has written about it many times. I asked Walter the other day to answer Andrew's rhetorical question. Here is what he wrote:

Full-throated support for hardline Israeli positions is a populist position in American politics -- like full-throated support for a fence on the Mexican border.  It is a foreign policy idea that makes elites queasy and that they try to steer away from, but support for it is so strong in public opinion, and therefore in Congress, that presidents have to figure out how to work with this force rather than taking it on directly.  

Lobby groups like AIPAC play a role, because most politicians do not want to be branded "anti-Israel" by AIPAC.  The reason is that getting called anti-Israel by AIPAC weakens your support among pro-Israel gentiles.  But if gentiles don't support hardline Israeli positions (like releasing convicted spy Jonathan Pollard), all the alleged mighty power of the Israel Lobby vanishes in a heartbeat.  

The Israel Lobby is all powerful when it has gentile public opinion behind it; it is a much weaker creature when it doesn't.  What Netanyahu demonstrated in Congress was not that he has the backing of the Israel Lobby.  It was something much more important and, depending on your viewpoint, more alarming: he has the backing of the American people.