The precipitous decline could force the government to cut back popular benefits, like subsidies for gasoline and utilities and the $12 monthly cash benefit that many Iranians have come to consider a birthright. And it could have even broader effects, possibly sending the economy into recession.

“Oil prices are bound to go down even further, and already the government faces big financial shortcomings,” said Jamshid Edalatian, an economist and member of the chamber of commerce in Tehran, which is close to the government. “We need this nuclear deal, or we will face very hard times.”

Mr. Rouhani and his chief negotiator, the foreign minister, Mohammad Javad Zarif, seem acutely attuned to the damage done by sanctions. In an interview in Vienna in July, and again in appearances in New York last month, Mr. Zarif talked about the cost to Iran — while insisting that the country’s reaction to pressure had been to build more centrifuges and a larger nuclear infrastructure.

“What has this accomplished for you?” he asked his largely American audiences.

But it is far from clear that Ayatollah Ali Khamenei, the supreme leader of Iran, shares that view. Traditionally, he has been less focused on Iran’s economic future than on its status as a regional and world player. If Mr. Zarif goes home with a deal, American officials say, there is no guarantee that Ayatollah Khamenei will make a cost-benefit analysis about whether to approve it.

“The decision is the supreme leader’s to make, and we still don’t understand much about his calculus,” one senior American official said, declining to speak on the record about the talks.

But the government has quietly begun to take measures to ease the losses in oil income, which are only now beginning to be felt since Iran typically sells its oil in three-month contracts.