On the eve of a crucial visit to the White House by Prime Minister Benjamin Netanyahu of Israel, that country’s most powerful American advocates are mounting an extraordinary public campaign to pressure President Obama into hardening American policy toward Iran over its nuclear program.

From the corridors of Congress to a gathering of nearly 14,000 American Jews and other supporters of Israel here this weekend, Mr. Obama is being buffeted by demands that the United States be more aggressive toward Iran and more forthright in supporting Israel in its own confrontation with Tehran.

While defenders of Israel rally every year at the meeting of the pro-Israel lobbying group, the American Israel Public Affairs Committee, this year’s gathering has been supercharged by a convergence of election-year politics, a deepening nuclear showdown and the often-fraught relationship between the president and the Israeli prime minister.

Mr. Obama and Mr. Netanyahu will both speak to the group, known as Aipac, as will the three leading Republican presidential candidates, who will appear via satellite from the campaign trail on the morning of Super Tuesday. Republicans have seized on Iran’s nuclear ambitions to accuse Mr. Obama of being weak in backing a staunch ally and in confronting a bitter foe.

The pressure from an often-hostile Congress is also mounting. A group of influential senators, fresh from a meeting with Mr. Netanyahu in Jerusalem, has called on Mr. Obama to lay down sharper criteria, known as “red lines,” about when to act against Iran’s nuclear ambitions.

“We’re saying to the administration, ‘You’ve got a problem; let’s fix it, let’s get back on message,’ ” said Senator Lindsey Graham, Republican of South Carolina, who took part in the meeting with Mr. Netanyahu and said the Israeli leader vented frustration at what he viewed as mixed messages from Washington.

“It’s not just about the Jewish vote and 2012,” Mr. Graham added. “It’s about reassuring people who want to avoid war that the United States will do what’s necessary.”

To give teeth to the deterrent threat against Iran, Israel and its backers want Mr. Obama to stop urging restraint on Israel and to be more explicit about the circumstances under which the United States itself would carry out a strike.

Specifically, Israeli officials are demanding that Iran agree to halt all its enrichment of uranium in the country, and that the suspension be verified by United Nations inspectors, before the West resumes negotiations with Tehran on its nuclear program.

The White House has rejected that demand, Israeli and American officials said on Friday, arguing that Iran would never agree to a blanket ban upfront, and to insist on it would doom negotiations before they even began. The administration insists that Mr. Obama will stick to his policy, which is focused on using economic sanctions to force the Iranian government to give up its nuclear ambitions, with military action as a last resort.

Despite the position of the Israelis and Aipac, the American intelligence agencies continue to say that there is no evidence that Iran has made a final decision to pursue a nuclear weapon. Recent assessments by American spy agencies have reaffirmed intelligence findings in 2007 and 2010 that concluded that Iran had abandoned its nuclear weapons program.

In his tone, at least, Mr. Obama is working to reassure Israel. In an interview published on Friday, Mr. Obama reiterated his pledge to prevent Iran from acquiring a nuclear weapon — with force, if necessary — and ruled out a policy of accepting but seeking to contain a nuclear-armed Iran. The Israeli government, he said, recognizes that “as president of the United States, I don’t bluff.”

The White House’s choice of interviewer — Jeffrey Goldberg, a national correspondent for the magazine The Atlantic — was carefully calculated. Mr. Goldberg is closely read among Jews in America; in 2010, he wrote an article exploring the situations under which Israel would attack Iran.

American Jews are anything but monolithic. More dovish groups, like J Street, are trying to make a case against a pre-emptive Israeli strike. But for the next few days, Aipac will set the tone for an intense debate over the Iranian nuclear threat.

Mr. Obama will not lay down new red lines on Iran, even if he discusses them with Mr. Netanyahu, administration officials said. And he is not ready to accept a central part of Israel’s strategic calculation: that an attack on Iran’s nuclear facilities would be warranted to stop it from gaining the capability to build a nuclear weapon, rather than later, to stop it from actually manufacturing one.

In the interview, Mr. Obama warned Israel of the consequences of a strike and said that it would delay but not prevent Iran from acquiring a weapon. He also said he did not know how the American public would react.

Israel’s supporters said they believed that a majority of Americans would support an Israeli military strike against Iran. But polling data paints a murkier picture: while close to 50 percent of Americans say in several polls that they would support Israel, a slightly larger number say they would stay neutral. In some surveys, there is strong support for continuing diplomacy.

Supporters of Israel argue that in the American news media, Iran’s nuclear program has been wrongly framed as Israel’s problem, rather than as a threat to the security of the whole world.

“This is about the devastating impact on U.S. and Western security of a nuclear-armed Iran bent on bullying the region into submission,” said Josh Block, a former spokesman for Aipac.

Turnout for this year’s Aipac conference is expected to surpass all previous records. And the roster of speakers attests to the group’s drawing power. In addition to Mr. Obama, Defense Secretary Leon E. Panetta will speak, as will Congressional leaders including Senator Mitch McConnell, the chamber’s Republican leader, and former Speaker Nancy Pelosi, the Democratic leader in the House.

On Tuesday, the screens in the Washington convention center will light up with the Republican presidential contenders Mitt Romney, Newt Gingrich and Rick Santorum, who are likely to fault Mr. Obama as not doing enough to prevent Iran from getting a weapon.

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