(Reuters) - Ever since their first awkward encounter - a hastily arranged meeting in a custodian´s office at a Washington airport in 2007 - Iran has been one of the few issues on which Barack Obama and Benjamin Netanyahu have been able to find some common ground.

Nearly five years ago, neither man was yet in power but both hoped to be, and though they were very different politicians they grabbed the opportunity to size each other up when their paths crossed.

The Israeli right-winger came across, at first, as strident in his views, while the newly declared Democratic presidential candidate seemed wary. But when Netanyahu insisted on the urgent need to do more to isolate Iran economically and Obama said "tell me more," the mood suddenly brightened, according to one account of the meeting.

It was part of what Netanyahu, who first served as prime minister from 1996 to 1999, has described as a 15-year personal effort to "broaden as much as possible the international front against Iran," a foe that has called for Israel´s destruction.

Obama, then a first-term senator, would go on to introduce an Iran divestment bill in Congress on the way to winning the White House in the 2008 election.

Now, with Obama and Netanyahu due to meet in Washington on March 5, the Iranian nuclear standoff will again top the agenda. But this time, a trust deficit between the two leaders could make it harder to decide what action to take against the Islamic Republic over its nuclear program.

The Obama administration, increasingly concerned about the lack of any assurance from Israel that it would consult Washington before launching strikes on Iran´s nuclear sites, has scrambled in recent weeks to convince Israeli leaders to give sanctions and diplomacy more time to work, U.S. officials say.

Israel has been listening - but after a series of high-level U.S. visits there is no sign it has been swayed.

Defense Minister Ehud Barak, who along with Netanyahu met U.S. National Security Adviser Tom Donilon last week, complained privately afterward that Washington is lobbying for a delay in any Israeli attack on Iran while time is running out for such a strike to be effective, Israeli political sources said.

Barak has spoken publicly of an Iranian "zone of immunity" to aerial attack, a reference to the start of additional uranium enrichment at a remote site believed to be buried beneath 80 meters (265 feet) of rock and soil near the city of Qom.

Donilon´s visit to Israel coincided with a cautionary note from General Martin Dempsey, chairman of the U.S. joint chiefs of staff, who told CNN it would be "premature to exclusively decide that the time for a military option was upon us."

The United States, Dempsey said, has counseled Israel "that it´s not prudent at this point to decide to attack Iran." He said sanctions were beginning to have an effect and it is still unclear whether Tehran would choose to make a nuclear weapon.

Obama and top aides have said they do not believe Israel has made a decision to attack Iran even as they caution about devastating consequences in the Middle East - and potentially around the globe - if it does so.

U.S. intelligence sources say they would expect little or no advance notice from Israel, except possibly as a courtesy call when any bombing mission is at the point of no return. But one line of thinking within the Obama administration is that this might be best for the United States since any sign of complicity would inflame the Muslim world.

"When it comes to something that the Israeli government considers essential to Israel´s security, they will take whatever action they deem necessary, even if there is a level of disagreement with other countries, including the United States," said Michael Herzog, a former chief of staff to Barak and now an international fellow at the Washington Institute for Near East policy.

Obama and his national security team have yet to determine how the United States would respond if Israel does attack Iran, one U.S. official said. But the growing chorus of warnings from Washington - Israel´s biggest source of military assistance - serves as a stark message of the potential fallout in relations between the two longtime allies.

The debate over the possibility of an Israeli strike has exposed an important difference of opinion over Iran, which says it is enriching uranium for peaceful purposes.

"We are of the opinion that the Iranian regime is a rational actor," Dempsey said in the CNN interview. "And it´s for that reason, I think, that we think the current path we´re on is the most prudent path at this point."

Netanyahu has made clear he believes that kind of thinking is wrong- headed.

"Since the dawn of the nuclear age, we have not had a fanatic regime that might put its zealotry above its self-interest," he told The Atlantic in 2009. "People say that they´ll behave like any other nuclear power. Can you take the risk? Can you assume that?"

ELECTION OPPORTUNITY

An Israeli strike ahead of the November 6 U.S. elections would put Obama in a serious political bind.

Already defending himself against Republican accusations that he has been too tough on Israel and not tough enough on Iran, he would be reluctant, at least initially, to come down hard on Netanyahu for fear of undercutting support among Jewish voters and other pro-Israel constituencies as he seeks re-election.

It´s that perceived window of opportunity for Israel to strike at a time when incumbent candidate Obama might be shy about challenging Netanyahu that has helped to fuel speculation of an Israeli attack soon.

But for Netanyahu to go ahead with an attack in defiance of Washington, he would risk not only damaging his country´s most crucial alliance but also face the near-certain prospect of Iranian retaliation with no immediate U.S. military help - or even a commitment to provide any.

More likely, Obama and Netanyahu will try to keep their differences behind closed doors and present a united front against Iran in next month´s talks.

Any further public rift between the two leaders, who will meet a day before the Super Tuesday voting contests in which 10 states hold presidential primaries or caucuses, would likely be seized upon by Republican candidates looking for ammunition against Obama.

And, for the second straight year, Netanyahu will be able to emerge from any White House chill into the warm embrace of the powerful pro- Israel lobby, the American Israel Public Affairs Committee (AIPAC), whose annual convention he will address in Washington.

But Israel, in weighing military action, faces the risk of a backlash from Congress and the American public if oil prices spike during a still-fragile economic recovery or if the United States is hit by revenge attacks on its interests around the world.

"It´s the law of unintended consequences," said an outside expert who advises the White House on national security. "This could lead to the first real reassessment in a generation of how America and Americans feel about Israel."

One American Jewish leader who knows both leaders played down the prospects of any dramatic shift in U.S.-Israeli relations.

Malcolm Hoenlein, executive vice chairman of the Conference of Presidents of Major American Jewish Organizations, told Reuters that strong bipartisan understanding for what he called Israel´s "responsibility to its citizens" meant that "nothing" would happen to ties between the two countries.

Last May, Netanyahu received 29 standing ovations when he addressed a joint meeting of Congress at the invitation of its Republican leadership. In the run-up to the November U.S. election, a senior legislator of his Likud party has been active in cultivating relations with top Republicans.

BLUFF?

Though U.S. officials have no reason to believe Israel is bluffing, some both inside and outside the administration suspect that Netanyahu is overstating the immediate danger of an Iranian nuclear "break-out."

Netanyahu, they say, may be seeking to pressure the United States and its European partners to move further on new oil-related sanctions, put enough of a scare into China and Russia to get them to ease resistance to tighter enforcement and extract a firmer U.S. commitment to military action if Tehran takes concrete steps toward bomb-making.

But even if his top generals and intelligence chiefs advise that it is time to act, questions remain whether Netanyahu, who lacks the extensive military resume of most of his predecessors, will be ready to do so, especially if it means going it alone without the United States.

An Israeli security source said that unlike Netanyahu´s predecessor Ehud Olmert, who conducted wars in the Gaza Strip and Lebanon and ordered the bombing of a suspected Syrian nuclear reactor during his 2006-2009 tenure, Israel´s current leader finds it hard to decide on risky operational matters.

For his part, Obama will be hesitant at this point to go further than his mantra that "all options are on the table" in dealing with Iran, and is likely to make clear to Netanyahu that without international legitimacy unilateral military action could backfire on Israel and lead to diplomatic isolation.

Moreover, the consensus in the U.S. defense community is that Israel, acting alone militarily, would only be able to slow Iran´s nuclear progress by months or possibly a couple of years.

That assessment is echoed by Israeli security officials, though they argue that their armed forces´ capabilities may have been underestimated - even by the friendly, informed Americans.

They note that Israel destroyed Iraq´s atomic reactor in 1981 knowing that this would only postpone Saddam Hussein´s quest for a bomb. Kept in the dark about the tactically audacious sortie, Washington responded angrily, at first. But it later thanked Israel for removing a potential Iraqi threat.

"The IDF (Israel Defense Forces) has been preparing its capabilities for years," chief of staff Lieutenant-General Benny Gantz said, without elaborating, in February 18 comments to Israeli reporters, when asked about the prospects for an imminent war on Iran.

Israel lacks heavy long-range air force bombers, but its advanced F- 15 and F-16 warplanes could hit sites in western Iran and further inland with air-to-air refueling and by using stealth technology to overfly hostile Arab nations.

It could also launch ballistic Jericho missiles with conventional warheads at Iran, according to a 2009 report by the Center for Strategic and International Studies in Washington.

Commandos might be deployed to spot targets and possibly launch covert attacks. Drones could assist in surveillance and possibly drop bombs of their own. Barak has said he believes the home front would suffer "maybe not even 500 dead" if Iran or its allies in Lebanon and Gaza retaliate with missile barrages.

Complicating matters is a basic lack of trust between the Obama administration and Netanyahu´s government, born in part out of the president´s earlier failed efforts to jumpstart Israeli-Palestinian peacemaking by pressuring Israel to freeze Jewish settlement expansion.

The last time Obama and Netanyahu met at the White House, in May, the Israeli leader bluntly took the president to task in remarks to reporters in the Oval Office, lecturing him on Jewish history and flatly rejecting his proposal that Israel´s 1967 borders be the basis for negotiations on creating a Palestinian state. Obama was furious and relations hit rock bottom.

Little more than a year before, Israel had announced a major new settlement expansion in East Jerusalem - a move that embarrassed Vice President Joe Biden during a visit - and Obama ordered Secretary of State Hillary Clinton to call Netanyahu and dress him down.

Not long afterwards, Obama walked out of tense talks with Netanyahu at the White House and left the Israeli prime minister cooling his heels while he had dinner with his family - treatment widely interpreted as a snub by Israeli media.

Frosty relations between the two leaders have thawed somewhat over the past year as Obama has taken a tougher line on Iran sanctions while refraining from any new Middle East peace drives. Obama also scored points with Israelis for opposing a Palestinian bid for U.N. statehood recognition last September.

"Open lines and security channels have brought the relationship to a particularly good point and at the same time there hasn´t been tension of late on other issues," a senior administration official said.

But some Obama aides remain suspicious of Netanyahu´s motives. They are convinced that he would prefer to see a Republican take control of the White House in 2013 for fear that Obama´s re-election would give him a freer hand to push anew for Israeli concessions to the Palestinians during a second term.

And any look at the Iranian equation cannot ignore the Holocaust factor - the alarm-ringing "never again" theme Netanyahu invokes in speech after speech about the existential threat that Israel, widely believed to be the Middle East´s only nuclear power, would face if Iran got the bomb.

Those who claim to know Netanyahu well say he means what he says; it is his job to ensure the Jewish state´s survival. He has made clear that in addition to the Iranian threat, he sees Israel at risk from the deep uncertainty sown by the Arab spring uprisings, especially with the toppling of Egyptian President Hosni Mubarak, who was seen by Israel as a guardian of its peace treaty with Egypt.

An address to Israel´s parliament in January on the annual International Holocaust Remembrance Day could easily be tweaked into the kind of statement the government might issue as Israeli planes head home from their Iranian bombing missions.

"We cannot bury our heads in the sand. The Iranian regime openly calls for the destruction of the State of Israel; it is planning the destruction of Israel; and it is working to destroy Israel," he said.

"In the end, with regards to threats to our very existence, we cannot abandon our future to the hands of others. With regard to our fate, our duty is to rely on ourselves alone." (Additional reporting by Dan Williams in Jerusalem and Mark Hosenball in Washington; editing by Janet McBride) (© Thomson Reuters 2012. 02/23/12)