With potential candidates for next year's US presidential elections starting to declare their hand, the chief donors on both sides of the political divide appear to have one issue uppermost in mind: Israel.

Among Republican hopefuls, there has been especially intense pressure to prove their unwavering support for the right-wing Israeli prime minister, Binyamin Netanyahu.

Reports this week suggested that one leading contender, Jeb Bush, brother of former President George W Bush, had become an early casualty among Republicans trying to prove their pro-Israel credentials.

The National Review reported that Bush was considered "a dead man politically" after losing the backing of the Republican party's kingmaker, billionaire casino magnate Sheldon Adelson.

Adelson is said to have been infuriated by comments from Bush's foreign policy adviser, James Baker, a former US secretary of state. Baker criticised Netanyahu in March at the annual conference of J-Street, a liberal Israel lobby group.