President Obama urged Israel and its most ardent American supporters Sunday to refrain from bellicose rhetoric toward Iran and to allow time for stiff economic sanctions to work against the Islamic republic’s nuclear ambitions.

As threats of an Israeli strike against Iran’s nuclear program increase, Obama argued that a military operation now would only strengthen Iran’s fragile diplomatic position and would fail to end its uranium enrichment program permanently.

Acknowledging that Iran’s clerical leadership may not respond to economic pressure, Obama assured the large audience of concerned Israeli supporters that he is willing to use “all elements of American power” to prevent the Islamic republic from developing a nuclear weapon. But, he said, diplomacy must first be allowed to run its course.

“For the sake of Israel’s security, America’s security, and the peace and security of the world, now is not the time for bluster,” Obama told the annual policy conference of the American Israel Public Affairs Committee, or AIPAC, a powerful lobbying group. “Now is the time to let our increased pressure sink in, and to sustain the broad international coalition that we have built.”

Obama’s public argument for patience stands as a likely preview of the message he will deliver privately Monday when he meets with Israeli Prime Minister Benjamin Netanyahu at the White House.

In a statement following Obama’s remarks, Netanyahu said he “very much appreciated” the president’s “position that Iran must not be allowed to develop nuclear weapons and that all options are on the table.”

He added: “Perhaps most important of all, I appreciated the fact that he said that Israel must be able to defend itself, by itself, against any threat.”

Obama’s AIPAC speech begins a critical week for his diplomacy to contain Iran’s nuclear ambitions and assure Israel, America’s closest ally in the Middle East, that he can be trusted to act in its security interests.

Israel has concluded that Iran’s leadership has decided to pursue a nuclear weapon, and international inspectors have uncovered evidence to suggest a military intent for a program that Iran claims is meant only for civilian power purposes.

The Obama administration is not convinced that Iran’s leaders have decided to develop a weapon, although U.S. diplomats have worked with European allies to implement oil and banking sanctions against Iran until it gives up its uranium enrichment program.

Those sanctions will take full effect this summer, and administration officials have said that their intent, in part, is to foment public unrest inside Iran that may force the country’s leadership to rethink the value of its nuclear program.

Obama is managing the Iranian nuclear issue during an election year when his Republican rivals have called his leadership abroad weak and his support for Israel suspect. He is also facing the political threat posed by rising gasoline prices, which analysts say could skyrocket if war breaks out between Israel and Iran in the oil-rich Persian Gulf region.

Campaigning Sunday in Georgia, Republican presidential candidate Mitt Romney said: “If Barack Obama gets reelected, Iran will have a nuclear weapon and the world will change.” The Republican presidential candidates are scheduled to address the AIPAC meeting Tuesday by videoconference from the campaign trail.

A Pew Research Center poll last month found that a narrow majority of Americans believe the United States should remain neutral in a war between Iran and Israel. Nearly 40 percent said the United States should back Israel, the poll found, in results that broke sharply along partisan lines. By a nearly 2-1 ratio, Republicans said the United States should take Israel’s side in the war.

The Obama administration does not want a war with Iran, at least not until sanctions have been given more time to work. Summarizing his appeal Sunday, Obama said: “I firmly believe that an opportunity remains for diplomacy – backed by pressure – to succeed.”

Obama used his remarks to make the strongest defense to date of his support for Israel, including his opposition to the Palestinian bid for statehood at the United Nations last fall. He cited the increase in military aid to Israel under his administration, stronger intelligence sharing and tight cooperation on antimissile systems.

Much of the tension between Obama and Netanyahu, which Republican candidates have highlighted on the campaign trail, has centered on the Israeli-Palestinian peace issue.

Obama has urged Israel to cease settlement construction in the occupied territories and to begin direct peace talks with Palestinian leaders, who have placed their own conditions on beginning negotiations toward a two-state solution to the conflict. On Sunday, he said he would not “apologize” for urging the two sides to reach a peace agreement, arguing that Israel’s leaders also understand the value of doing so.

“There should not be a shred of doubt by now that when the chips are down, I have Israel’s back,” Obama said to applause. “So if during this political season you hear some question my administration’s support for Israel, remember that it’s not backed up by the facts. And remember that the U.S.-Israel relationship is simply too important to be distorted by partisan politics.”

An estimated 13,000 people attended the opening day of AIPAC’s annual conference, and Obama received a largely warm reception. Shimon Peres, Israel’s president and former prime minister, said during introductory remarks that “Israel has a friend in the White House.”

In addressing the Iranian nuclear program, Obama said he understood the “basic truth” that “no Israeli government can tolerate a nuclear weapon in the hands of a regime that denies the Holocaust, threatens to wipe Israel off the map, and sponsors terrorist groups committed to Israel’s destruction.”

He said preventing Iran from obtaining a nuclear weapon is also in the United States’ national security interests.

Administration officials fear that a nuclear weapon in the hands of Iran, a Shiite Muslim theocracy, would set off an arms race in the Middle East, as threatened Sunni nations move to develop or buy one of their own.

Israel has its own undeclared nuclear program, including a large weapons arsenal. But Obama has declined to call on Israel’s leaders to declare the program and join the non-proliferation obligations that Iran has pledged to abide by.

“Because of our efforts, Iran is under greater pressure than ever before,” Obama said, citing the coalition of European and Asian nations that have signed on to sanctions. “That is where we are today. Iran is isolated, its leadership divided and under pressure. And the Arab Spring has only increased these trends, as the hypocrisy of the Iranian regime is exposed and its ally, the Assad regime, is crumbling.”

Obama, who has brought the U.S. war in Iraq to an end and is bringing soldiers home from Afghanistan, also acknowledged that “as president and commander in chief, I have a deeply held preference for peace over war.”

“We all prefer to resolve this issue diplomatically,” Obama told the audience. “Having said that, Iran’s leaders should have no doubt about the resolve of the United States, just as they should not doubt Israel’s sovereign right to make its own decisions about what is required to meet its security needs.”