I'd forgotten about it, but M.J. Rosenberg's post here reminded me of the existence of Michael Kinsley's hilarious 2003 tour of anti-semitism on the AIPAC website:

It is my sad duty to report that this form of anti-Semitism seems to have infected one of the most prominent and respected—one might even say influential—organizations in Washington. This organization claims that "America's pro-Israel lobby"—and we all know what "pro-Israel" is a euphemism for—has tentacles at every level of government and society. On its Web site, this organization paints a lurid picture of Zionists spreading their party line and even indoctrinating children. And yes, this organization claims that the influence of the Zionist lobby is essential to explaining the pro-Israel tilt of U.S. policy in the Middle East. It asserts that the top item on the Zionist "agenda" is curbing the power of Saddam Hussein.

On a more serious note, it occurs to me that some of these disputes about how powerful some lobby is or isn't in DC get a little hard to resolve because the perception of power is, itself, an important source of power which creates a lot of ambiguity as to what it even means to ask how powerful a group "really" is.