“What’s all this talk, that we will decide alone on our fate and that we won’t take anybody else into consideration?” said Mr. Olmert, who is expected to make Mr. Netanyahu’s relationship with Mr. Obama a mainstay of his campaign if he runs. “Can someone please explain to me with which airplanes we will attack if we decide to attack alone, against the opinion of others — airplanes that we built here in Israel? With which bombs will we bomb, bombs that we made by ourselves? With which special technologies will we do it, those that we made by ourselves or those that we received from other sources?”

But when shown a video of Mr. Olmert’s retort, Mr. Netanyahu was not cowed. “If what I just heard is that on this matter which threatens our very existence, we should just say, we should just hand the keys over to the Americans and tell them, ‘You decide whether or not to destroy this project, which threatens our very existence,’ well, that’s one possible approach, but it’s not my approach,” he said. “My approach is that if we can have others take care of it, or if we can get to a point where no one has to, that’s fine; but if we have no choice and we find ourselves with our backs against the wall, then we will do what we have to do in order to defend ourselves.”

After years in which Mr. Netanyahu and Mr. Barak pursued the Iranian threat in close partnership, the prime minister now seems virtually alone in his defiant stance, as other leaders attempt to distinguish their positions ahead of Israeli elections on Jan. 22. While Mr. Netanyahu said in his Sept. 27 speech at the United Nations that the critical moment for preventing Iran from developing a weapon would most likely come next spring, Mr. Barak last week pushed the timetable back further, and offered a new explanation of Israel’s reduced sense of urgency.

The crux of Mr. Barak’s argument, made in an interview with Britain’s Daily Telegraph, was based on reports by the International Atomic Energy Agency, the most recent in August, showing that Iran had 189 kilograms, about 416 pounds, of uranium enriched to the 20 percent level — from which it could relatively easily be further enriched to weapons grade. Roughly half of that was diverted to civilian use in a form that could not be easily turned into bomb fuel. But Iran has continued production and by most estimates, at current rates, would have roughly a bomb’s worth by next summer.

That “allows contemplating delaying the moment of truth by 8 to 10 months,” Mr. Barak said.

But several high-ranking Israeli officials and analysts said that Mr. Barak’s explanation was overly simplistic. While the diversion was clearly a factor, they said, it was not a new development: the nuclear agency had reported a similar transfer of enriched uranium in May, and that had hardly cooled the rhetoric of either Mr. Barak or Mr. Netanyahu through the summer. And both men have long warned of secret centrifuges that could be spinning without outside knowledge, enabling rapid replenishment of the enriched stockpile.

“Netanyahu backed away because he was getting the message that he was going too far and this could do damage, this was not helpful either to Israel or to stopping Iran,” said Emily Landau, an Iran expert at the Institute for National Security Studies at Tel Aviv University. “It might be easier for Barak to now say that it’s because of the technical issue, but it’s not a real issue. Relations with the United States is a much more substantial, real issue, but it’s more difficult to give that as your explanation.”