All major players in Tehran, whatever their ideological leanings, are pragmatic enough to know that only by saving the tattered 2015 nuclear deal can the Islamic republic free itself from crippling US sanctions, they argue.

The decision to try to revive the agreement, struck under moderate President Hassan Rouhani, now "transcends factional struggles," said Clement Therme, a researcher at the European University Institute in Florence, Italy.

"It is a trade-off, between the survival of the regime through improving the deteriorated economic situation, and the desire to preserve the status quo on the political level," he told AFP.

Analyst Henry Rome of the New York-based Eurasia Group said Tehran appears determined to revive the deal torpedoed by former US president Donald Trump because "for Iran, sanctions relief is a strategic necessity".

Ultimate political power in Iran rests with the supreme leader, Ayatollah Ali Khamenei, who gave the green light to both the original agreement with a group of world powers, and to efforts since April to revive it.

The landmark accord offered Iran crucial sanctions relief in return for limits on what it says is a civilian nuclear programme -- but it has been on life support since Trump withdrew the United States from it in 2018.

Trump's economic and diplomatic "maximum pressure" campaign has plunged Iran into its deepest economic turmoil in decades, and led it to progressively walk back from most of the limits on its nuclear programme.

Trump's successor Joe Biden has since backed diplomatic efforts to revive the accord, and US officials have indirectly joined talks in Vienna between Iran and the other parties, Britain, China, France, Germany and Russia.

- 'Reap political benefits' -

Rouhani, the architect of the deal, has in recent years weathered heavy criticism from the conservative camp for having trusted the West in negotiating an agreement that Trump then ripped up.

Ultraconservative candidate Ebrahim Raisi, who has pledged to resolve Iran's economic crisis, is now seen as the frontrunner to replace Rouhani in the June 18 election, after many other candidates were barred from running.

Rouhani, who has served the maximum of two consecutive four-year terms allowed under the constitution, remains in office until August, and hopes for a deal in Vienna before then.

A spokesman for his outgoing government, Ali Rabii, also stressed that the election will not change Iran's basic position in the complex talks.

"The nuclear issue (is the subject) of a consensus within the Islamic Republic," he said Tuesday. "It is therefore not linked to the country's internal developments and is handled by high-level bodies."

Iran has suffered badly since Trump's unilateral US withdrawal and pressure on European parties to also economically isolate Iran.

Rather than enjoying an influx of foreign investment, Iran saw crucial oil sales dry up and was shut out of the international financial system.

The country of 83 million has been battered by galloping inflation and rising unemployment which have stoked repeated waves of unrest on the streets.

Throughout, Iran has stayed in the agreement, even as it has taken steps away from its commitments, such as increasing uranium enrichment and suspending certain inspections by the International Atomic Energy Agency.

Now that a Biden presidency promises a chance for a turnaround, Tehran is unlikely to abandon its policy of "strategic patience", say analysts.

The Eurasia Group said in a note that Raisi, the likely election winner, "has expressed support for a return to the nuclear accord, and he would very likely follow through on its implementation as president".

If a deal is not agreed before August, "he would probably move swiftly to wrap up negotiations" after, it said, hoping to "reap significant political benefits" from renewed oil sales, access to frozen foreign exchange reserves and revived growth.

"This would give Raisi a substantial cushion in his first year or two in office."