At one point last week the simmering tension between the two negotiators boiled over when Mr. Zarif felt his American counterpart was pressing too hard. “Never threaten an Iranian!” he shouted. At the other end of the table, Sergey V. Lavrov, the Russian foreign minister, who has had his share of disputes with Mr. Kerry, tried to break the tension. “Or a Russian!” he said, as the room broke out in nervous laughter.

But during a break on one particularly discouraging March day in Lausanne, Switzerland, where negotiations were held before adjourning to Vienna, Mr. Zarif struck a different tone as he invoked the names of the key figures on two sides, including Vice President Joseph R. Biden Jr. and the top energy officials of the United States and Iran, Ernest J. Moniz and Ali Akbar Salehi.

“We are not going to have another time in history when there is an Obama and a Biden and a Kerry and a Moniz again,” he said, according to notes of the conversation. “And there may be no Rouhani, Zarif and Salehi.”