It is said to be the most powerful interest group in Washington DC - but what is the Israel lobby? And how exactly does it operate?

As the race to the White House heats up, there is one policy on which all the presidential candidates can agree - the US relationship with Israel.

Israel is the world's largest recipient of US aid, taking in roughly $3bn in direct assistance every year.

It is an aid relationship unlike any other in the world - and by far the most generous foreign aid programme ever between any two countries.

Yet for decades, the US has criticised Israel for its policy of building settlements on occupied Palestinian land.

Every president from Jimmy Carter to George Bush has warned Israel to put a halt to its settlement expansion - and yet the settlements continue to go up, and the aid money continues to pour in.

How could this be? Many have attributed it to the so-called Israel lobby - a powerful coalition of interest groups working to promote a pro-Israel agenda in Washington DC.

For Israel's friends on Capitol Hill it can mean tens - sometimes hundreds - of thousands of dollars worth of campaign contributions from political action committees.

For Israel's enemies? It can mean the end of a political career.

So just how powerful is the pro-Israel lobby in America, and where is US policy heading from here?

That is the question for this week's Inside USA.

This episode of Inside USA aired from Saturday, April 5, 2008.