The power of America's "Jewish lobby" is said to be legendary.
Commentators the world over refer to it, as though it were a well-established fact that US Jews wield far more influence than their numbers (2% of the population) would suggest.
But this presumed influence is also a delicate issue in the US, and is rarely analysed.
How does the lobby work? Is its power truly legendary, or just a legend?
Two US academics, John Mearsheimer of the University of Chicago and Stephen Walt of Harvard, have set out to answer those questions, and triggered a firestorm of controversy as a result.
Their book The Israel Lobby and US Foreign Policy, which builds on a 2006 article in the London Review of Books, says the reasons for US support for Israel need to be explained.
America spends $3bn a year in largely military assistance - one-sixth of its direct aid budget - to help a prosperous, nuclear-armed country, and strongly backs Israel in negotiations on Middle East peace.
But according to Mearsheimer and Walt, the US gets remarkably little in return.
They reject the argument that Israel is a key ally in America's "war on terror".
On the contrary, they contend, US patronage of Israel fuels militant anger - as well as fostering resentment in Arab countries that control vital oil supplies. (MORE)