As US presidential candidates battle it out to become the leader of the world's only superpower there is one subject on which they all, in public at least, agree - the US relationship with Israel.

To leading politicians on both sides of the partisan divide the special relationship is sacrosanct, largely due, critics say, to the power of pro-Israel lobby groups.

Those critics also say that pro-Israeli groups are set to play a major role in the forthcoming election battle, both in terms of funding candidates and by publicly criticising any candidate critical of Israel or the US's relationship with it.

John Mearsheimer, who alongside Stephen Walt is the author of a controversial series of articles and a recent book on the Israel lobby, told Al Jazeera: "Almost all of the major candidates are falling over themselves to demonstrate how deeply committed they are to America's special relationship with Israel.

"Hardly a word of criticism is directed at anything Israel does and that is due to the activities of the lobby."

What is the pro-Israel lobby?

The lobby is made up of dozens of pro-Israel political action committees that draw a large part of their support from the US Jewish community and provide funding to presidential candidates.

But Christian Zionists, who are among the most vociferous supporters of Israel in the US, also play a major role.

They believe that by strengthening and supporting the state of Israel, they are more likely to bring about the "second coming" of Jesus as prophesied in the Bible.

At the lobby's vanguard is the American-Israel Public Affairs Committee (Aipac), which works mainly in US congress.

It boasts its recent "victories" include the US decision to brand Iran's Revolutionary Guards as a terrorist organisation, securing US aid to Israel and freezing US aid to the Hamas-led Palestinian Authority in 2006.

Money and power

Defenders of the Israel lobby say that the views of presidential candidates are really a natural reflection of the views of most Americans and that it has little influence over elections.

But money talks in politics and the figures tell a different story.

The Centre for Responsive Politics (CRP), which monitors the role of money in US politics, says pro-Israeli groups and individuals have already donated more than $845,000 to presidential candidates in the 2008 campaign -  70 per cent of it to Democrats.

In the entire 2004 presidential campaign pro-Israel interests contributed at least $6.1 million to federal candidates and parties.

"Money translates into influence in Washington, so generally the interests that spend the most money are going to get the best access and results," says Massie Ritch, communications director at the CRP.

And it is outside of the presidential race and in congress, which holds the purse strings on the key area of aid to Israel, that the lobby makes its financial mark.

Aipac and other groups spent more than $1.5 million on federal lobbying in 2006 and more than $1.25 million in the first half of 2007, meaning that this year could be a record one for the lobby.

The pro-Israel lobby accounts for about one-quarter of all foreign policy lobbying on Capitol Hill, the CRP says.

Information flow

Arab efforts to put their case across are, in contrast, minimal.

The National Association of Arab-Americans and the American Arab Anti-Discrimination Committee reported spending just $80,000 on federal lobbying in 2006 and $40,000 in the first six months of 2007.

The financial power of the Israel lobby also allows it to provide one-sided information to US politicians not always familiar with the complexities of conflict in the Middle East.

Aipac provides educational trips to congressman and their staff - more trips than any other sponsor, according to the CRP.

"Members of congress and their staffs have been to Tel Aviv more often in recent years than they've been to Chicago," says Ritch.

Anti-Semitism charges

Aipac's defenders say that this is where the organisation plays an important role, as an information source for politicians - including US presidential candidates.

But critics say that pro-Israel lobby groups go much further - as John Mearsheimer says: "The lobby monitors what the candidates say very closely."

In March, Democratic candidate Barack Obama gave a speech in the key primary state of Iowa where he said: "Nobody is suffering more than the Palestinian people."

A local Aipac member immediately contacted the media to denounce the comment, describing it as "deeply troubling".

In July Jim Moran, a Democratic congressman who has criticised Aipac in the past, accused the organisation of pushing for war on Iraq.

Seventeen members of congress immediately wrote a letter to Moran condemning him and saying that his remarks "unfortunately fit the anti-Semitic stereotypes some have used historically used against Jews".

Eric Cantor, the house of representatives Republican deputy chief whip, reportedly went further and was quoted as as saying: "Unfortunately, Jim Moran has made it a habit now to lash out to the American-Jewish community.

"I think his remarks are reprehensible, I think his remarks are anachronistic, and hearken back to the day of Adolf Hitler."

In such a political climate it is easy to see why those seeking a job in the Oval Office are wary of speaking out for any change in the US relationship with Israel or against Aipac.

The charge of anti-Semitism is regularly used by the Israel and lobby and was one of the charges faced by John Mearsheimer.

"We are not anti-Semites and the book is not anti-Semitic," he says.

"Calling critics of Israeli policy or the US-Israel relationship is standard operating procedure for the lobby. It's the standard strategy they use to stifle criticism of Israel and to marginalise those critics."

Military action

Beyond the politics of elections, the lobby's critics say that pro-Israeli groups, after pushing for war on Iraq, are now advocating military action against Iran.

"If you look at who is pushing the US to use military force against Iran, the two driving forces are Israel and the Israel lobby," says Mearsheimer.

Jim Moran, in an interview with the Tikkun, a Jewish peace magazine, said US action against Iran is proposed only because it is a threat to Israel.

"No one's suggested that Iran is a potential threat to the United States," he told the magazine, "any more than Iraq could ever have been a threat to the United States."

"In effect, all the same groups and individuals who were pushing for war against Iraq are pushing for war against Iran."

Aipac, however, vehemently denies it is asking for anything other than sanctions.

"Aipac solely advocates sanctions as the best way to stop Iran from acquiring nuclear weapons or the capability to make them," Josh Block, an Aipac spokesman, told Al Jazeera.

Real friendship

Mearsheimer argues that the US needs to normalise its relationship with Israel, treating it more like the UK, Germany or India.

He and other critics, from both inside and outside the Jewish community in the US, argue that Israel also suffers from its privileged position in terms of US aid.

They believe that the Israel lobby's support in the US encourages Israel to act without fear of international sanction.

This has emboldened Israeli leaders to sanction the illegal occupation of Palestinian land, the building of settlements and for the Israeli military to carry out numerous human rights abuses.

"If these presidential candidates were real friends of Israel as they claim to be, they would not only be criticising Israel for its policies in the occupied territories ... they would be arguing that the US put significant pressure on Israel and the Palestinians to reach an agreement on a two-state solution," Mearsheimer told Al Jazeera.

"That's what a real friend would do."