WASHINGTON/JERUSALEM (Reuters) - Just days before what could be the most consequential meeting of U.S. and Israeli leaders in years, aides to President Barack Obama and Prime Minister Benjamin Netanyahu are scrambling to bridge stark differences over what Washington fears could be an Israeli attack on Iran's nuclear sites.

Further complicating Monday's White House talks is a trust deficit between the two men that has been magnified by mounting pressures of the U.S. presidential campaign. Obama's Republican foes are eager to paint him as too tough on Israel and too soft on Iran.

Netanyahu is coming to Washington to press Obama to more forcefully declare "red lines" that Iran must not cross in its nuclear program, Israeli officials say, even as speculation mounts that the Jewish state could act militarily on its own in coming months.

"If you don't want me to attack now, I want guarantees," an Israeli official quoted Netanyahu telling top Obama aides who visited Jerusalem last month. "If you're saying, 'we'll take care of you', you're not saying that clearly enough."

The White House has signaled that Obama, who has pledged to prevent Iran from building a nuclear weapon but has been vague on how far he is prepared to go, will resist pressure for a public policy shift.

Instead, amid growing signs that U.S.-led international sanctions are starting to take a toll on Iran, he will seek to persuade Netanyahu to hold off on any military strike to give those measures and diplomacy time to work, U.S. officials say.

But Israeli officials say they fear that time is running out for an effective Israeli attack as Iran buries its uranium enrichment program deeper underground.

Monday's meeting was supposed to have been a defining moment for the American and Israeli leaders, a chance to present a united front as international pressure on Iran intensifies.

Underscoring the gap between the two allies, the Israelis also complain that the Obama administration is undercutting the deterrent effect of their threat to use force by publicly questioning the timing and wisdom of airstrikes on Iran, which says its nuclear activities are for generating electricity.

Calls for a tougher approach on Iran are also coming from Republican presidential hopefuls, who see Obama as vulnerable on the issue as he seeks re-election and will seize on any public rift with Netanyahu.


Netanyahu will be pushing not only for Obama's acceptance of whatever action Israel decides to take but for stronger language against Iran that goes beyond the "all options are on the table" mantra on U.S. intentions, Israeli officials said.

Washington has been working to convince the Israelis that a go-it-alone attack would cause only a temporary setback to Tehran's nuclear ambitions while possibly plunging the already-volatile Middle East into chaos.

And Obama's aides insist that an explicit U.S. military threat would be counterproductive right now, especially due to the potential for further spikes in global oil prices and the risk that Tehran might backtrack on overtures seen as opening the door to renewed nuclear talks with world powers.

But a source close to the administration's thinking on Iran said the president might try to placate some of Netanyahu's concerns in private and could also pledge even more sanctions to tighten the vise on Tehran.

The White House has proposed the two leaders issue a joint statement after they meet, but the idea has yet to be firmed up, an Israeli official said. A show of solidarity on certain issues might help keep differences under wraps on others.

An administration official also would not rule out the possibility that Obama could harden some of his rhetoric on Iran when he addresses the largest U.S. pro-Israel lobby in Washington on Sunday, the day before he sees Netanyahu.

Despite that, U.S. officials doubt that Netanyahu will go as far as providing assurances that Israel will consult Washington - its biggest source of military assistance -- before launching any strikes on Iran, which has called for the destruction of the Jewish state.

Even if Obama privately reassures Netanyahu that the U.S. has the firepower to deliver a devastating blow to Iran's nuclear program further down the line, the Israelis have made clear they cannot rely on that commitment alone.

"Anyone who thinks that Israel is not going to make its own decision, particularly on an issue they view in existential terms, is kidding themselves," said Obama's former Middle East adviser, Dennis Ross.

One line of thinking within the Obama administration is that keeping it in the dark about any Israeli military plans might be best for the United States since any sign of complicity would inflame anti-American sentiment in the Muslim world.

But even without a direct U.S. role, there will be deep suspicion across the Middle East that Israel would not act without a green light from Washington.


Still, an Israeli strike ahead of the November 6 election would put Obama in a political bind. He would be reluctant to come down hard on Netanyahu for fear of undercutting support among Jewish voters and other pro-Israel constituencies.

Ross, who has advised both Democratic and Republican administrations spanning three decades, suggested that the "noise" from Israel over a possible strike was geared more toward pressuring the international community for further sanctions than foreshadowing an imminent attack on Iran.

"Now that it's an issue of the world against Iran, Israel likes it that way and would not be inclined to act precipitously," Ross said.

Still, it remains unclear whether Netanyahu will pay much heed to Obama's words of caution.

At their last Oval Office meeting a year ago, Netanyahu embarrassed Obama by lecturing him about Jewish history. But relations have thawed somewhat as Obama has taken a tougher line on Iran 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.

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.

(Editing by Anthony Boadle)

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