One of the most enduring myths about American politics, a helpfully all-encompassing theory that purports to explain a bedrock feature of US foreign policy - and laced with just the faintest hint of the world's oldest hatred - is that of the all-powerful Jewish lobby.

To outsiders, the spectacle of politicians lining up to pay homage to a lobby group that promotes the interests of one small country in the Middle East is proof positive of the disproportionate power wielded by Jews in American life.

Why else would presidential contenders rush every year to attend the annual meeting of the American Israel Public Affairs Committee (AIPAC)? Why else would they go to great lengths to ensure that they say nothing there that does not get the committee's approval?

The Jewish vote, after all, is pretty insignificant. Less than 3per cent of the population is Jewish - about the same number as Muslims. In New York and Florida the proportion is larger, but still in single figures. The Jewish population's direct political influence is probably further diminished by its tendencies to vote for Democratic candidates. In this, as in many other respects, Jews' voting behaviour is formed much more by their socio-economic conditions than any religious or foreign policy concerns.

If Jews wield little direct electoral clout, the reason for the power of the Israel lobby, say its critics, must be that it uses the financial and political muscle of American Jews to exercise a stranglehold on foreign policy debate. It requires politicians to commit America to uncritical support for Israel, irrespective of other US interests in the region.

There is a lot wrong with this idea. In a country as diverse as America, candidates are constantly trying to ensure that they are in the good graces of people of almost all faiths and traditions. Last week, John McCain was forced to disavow his support from an evangelical preacher who described Catholicism as a “godless theology of hate”.

But there is a bigger reason to object to the familiar characterisation of the Jewish lobby. AIPAC is undoubtedly one of the most effective lobbying organisations in Washington. But it succeeds because very large numbers of Americans share its aims, not because it somehow strongarms politicians into supporting it. Candidates want AIPAC's approval because they know that being seen as pro-Israel is central to their foreign policy credentials.

In opinion polls Americans express overwhelming support for Israel. They see it in kindred terms - a thriving democracy forged in an inhospitable climate. For Barack Obama in particular, dispelling doubts about his pro-Israel credentials is essential to winning the votes of most Americans.

Times Recommends

• Fundamentalist brew: faith and power
• Big guns wheeled out for the marginals
• Wanted: new merchants of light