For Israeli leaders, a public break with the United States is the third rail of politics. The possibility that Israelis might lose the support of the one nation that can guarantee their security awakens an existential dread that no politician can long survive. It is this factor, as much as any, that has restrained the Israelis from taking military action against Iran despite Tehran's efforts to build a nuclear-weapons capability. But now the possibility of such a break seems higher than it has in two decades. So it's no surprise that, as he prepares for his first meeting as prime minister with President Obama on May 18, Benjamin Netanyahu has been "fine-tuning" his hard-line positions on peace and Iran, as a senior Israeli official described it. "As we speak, there are meetings going on to make sure we have a success" at the summit, the official said.

In his televised speech Monday night to the annual conference of the American Israel Public Affairs Committee (AIPAC), the powerful lobbying group, Netanyahu took the hard edge off the messages he has been sending since last year: that any peace with the Palestinians will have to await a resolution of the Iran nuclear issue. "We are prepared to resume peace negotiations [with the Palestinians] without delay," Netanyahu said. But he added what he described as "two provisos." First, he said, "peace will not come without security." This sounds bland, but it appears to be code for a position that Netanyahu has been advancing for years and which he hammered home to Obama during their first visit last July, when the then-senator from Illinois visited as a presidential candidate: without addressing Iran's attempted rise as a nuclear-powered regional hegemon, there can be no security in any other area—especially the Palestinian conflict. As Uzi Arad, Netanyahu's national-security adviser, told me shortly after that meeting: "Should one fail to neutralize that Iranian threat now, it would undercut anything that would be achieved with the Palestinians, Syria or Lebanon … If you follow that logic, the current efforts to move on the Palestinian issue are pathetic, because they would not be worth the paper they're written on if Iran is not contained. If Iran became nuclear it would mean the victory of the militants in Hamas and Hizbullah and undercut the moderates."

The second proviso, Netanyahu said, is that the "Palestinians must recognize Israel as the Jewish state." This too is an ambiguous statement, but reading between the lines it almost certainly means that Netanyahu will not recognize any peace agreement that hands over the West Bank to the Palestinians as long as Hamas continues to wield the political power it does in the territories and refuses to recognize Israel (a position that Hamas leaders reiterated this week). It also means that, if talks do begin again, an issue that once was deemed "final status"—the right of Palestinians to return to Israel—is off the table.

To say that Netanyahu is "fine-tuning" his positions is a euphemistic way of saying that he doesn't like the message coming from top officials of the Obama administration, but he's not ready to commit political suicide. Already suspected by a large portion of the electorate, he can't afford to be seen to be on the outs with Obama. The Israelis, in other words, are going to do their best to avoid the perception that there is daylight between Jerusalem and Washington (that existential dread again). But the daylight is already streaming out.

Indications are that Obama intends to push all-out for a Palestinian state, and that he plans to negotiate in a broad-based way with Iran. The "tough love" message from Washington has emerged in a series of statements from top U.S. officials. Vice President Joe Biden, in his speech to AIPAC on Tuesday, said Israel "has to work for a two-state solution ... not build more settlements, dismantle existing outposts and allow Palestinians freedom of movement."

National-security adviser James Jones, in comments to a European counterpart reported by Haaretz—which cited a classified Israeli telegram relaying the conversation—said the Obama administration "will convince Israel to compromise on the Palestinian question." Jones reportedly added: "We will not push Israel under the wheels of a bus, but we will be more forceful toward Israel than we have been under Bush." (The National Security Council did not return a call asking for comment.) White House chief of staff Rahm Emanuel, in his own remarks to AIPAC delegates, sought to allay some concerns while laying down the new line. "Relations between Israel and the U.S. are unbreakable," Emanuel said. But he added: "This is the moment of truth for Israel and the Palestinians."

It just as likely a moment of truth for Israel and the United States. For the last eight years Washington acted mainly as an unswerving supporter of Israel's actions—some critics would say cheerleader—despite a few serious differences, such as the timing of the 2006 Palestinian elections. But the potential now exists for the most serious rupture of relations at least since 1989, when Secretary of State James Baker stunned AIPAC by calling on Israel to abandon its "unrealistic vision of a Greater Israel" that included Gaza and the West Bank.

The biggest potential flashpoint is Iran's nuclear program. The Obama administration is staying quiet about the new strategic approach it is developing—but make no mistake, it will be a dramatic departure. "Things are undergoing a strategic shift, but we don't know what the details are yet," says a top European official involved in talks with Tehran. According to a Syrian official who has been involved indirectly in the new effort at engagement—the Syrians have offered to act as "facilitators"­—Washington is mainly awaiting the results of Iran's June presidential election, in which Mahmoud Ahmadinejad is vying with several reformist candidates.