The July 2015 agreement to dissolve
It's the first major political test for moderate President
Rouhani's popularity has soared since the nuclear deal, which enjoys broad support among Iranians — particularly after the United States and other world powers lifted sanctions last month, raising hopes that the resumption of foreign investment will restore the country's economic health.
But his other reform efforts have been stymied by a parliament dominated by powerful conservatives, who fear the president and his supporters are too accommodating to the West.
Iran remains a theocracy in which religious hard-liners hold sway over the political process, and they have taken steps to ensure the nuclear deal doesn't tilt the balance of power too greatly in favor of Rouhani and his allies.
What exactly are Iranians voting on?
Voters will select members of the 290-seat parliament, or Majlis, which drafts laws and approves the president's Cabinet appointments. The current parliament has been a thorn in Rouhani's side, with a small group of hard-line opponents of the nuclear deal obstructing his agenda.
Perhaps even more crucial, voters will also cast ballots for the Assembly of Experts, a body of 88 Islamic jurists who serve eight-year terms. Its makeup has taken on greater importance in this vote because the next assembly could select the successor to supreme leader Ayatollah Ali Khamenei, who is 76 and believed to be in deteriorating health.
Put together, analysts say, it makes for the most consequential nonpresidential elections in Iran in decades.
How free are the elections?
Not very. The Guardian Council, a group of jurists and theologians who supervise the elections, disqualified more than half of the roughly 12,000 candidates who had registered to contest parliamentary seats.
The vast majority of those thrown out were reformists who oppose the religious hard-liners' four-decade grip on politics. In three-quarters of constituencies nationwide, the reformist camp was left without names to put on the ballot, forcing it into an alliance with moderates.
Even a grandson of the Ayatollah Ruhollah Khomeini, the leader of the 1979 Islamic Revolution, was barred from running — ostensibly because his moderate views and modern profile made him a possible leader of the reform movement.
"That was the entire design of the Guardian Council and the conservatives," said Ali Vaez, senior analyst with the International Crisis Group. "Their primary objective was to make sure the nuclear deal does not tilt the balance of power toward the reformists."
There were other signs Wednesday of a clampdown as riot police were deployed in central Tehran at the sites of reformist political rallies and police dispersed about 100 people from a wrestling stadium where one reformist event was due to be held.
What are the battle lines?
Even with the disqualifications, the vote has shaped up into a contentious referendum on the nuclear deal, pitting hard-liners and opponents of the agreement against reformists and their moderate allies, led by Rouhani.
Unlike in 2004 and 2012, when reformist groups boycotted polls after mass disqualifications, the pro-Rouhani group is hoping that a large turnout will work in its favor and is encouraging supporters to back its entire list of candidates.
They include people such as Ali Motahari, a well-known conservative who once railed against women wearing tight dresses in public but nonetheless casts himself as a political reformist. Voicing support for the nuclear deal, he told a rally at a Tehran football stadium this week to vote for "economic interaction with the world."
Ali Sedaghati, an accountant working for state-owned companies, said Rouhani had lifted the country out of a crisis that portended "war and more sanctions."
Rouhani's critics say Iranians were better off before the nuclear deal.
"The nuclear deal has not brought anything for the grass-roots and low-income people so far," said Fahime Rasti, a journalist allied with the hard-line camp.
What's the most likely outcome?
Polls indicate that moderate-reformist candidates will do well, and even if they don't gain a majority, the parliament is likely to be friendlier to Rouhani.
The president may still face resistance to efforts to make the country more socially and culturally liberal. Even his other promises, such as releasing political prisoners and jailed journalists, could remain difficult to implement over conservative objections.
But a strong electoral showing will buoy his economic agenda, particularly efforts to promote foreign investment, with a year and a half remaining before he faces reelection.
Could the elections affect implementation of the nuclear deal?
For now, there is little prospect of Iran turning its back on the agreement. A recent deal to sell Iranian oil to France and buy 118 Airbus jets was a sign of the benefits that could come to Iran's economy in a post-sanctions era.
Analysts say greater risk lies in the United States, where some Republican presidential candidates have said they would revoke the nuclear deal if elected.
"Both electoral politics and congressional efforts to undermine the deal could push the Iranian leadership to start taking measures of their own that would eventually weaken the agreement," Vaez said.
"There is no evidence on Iranian side that they want to give up on the deal. They're still hoping for the economic benefit, so it's too early to start thinking about undermining it."
Special correspondent Mostaghim reported from Tehran and Times staff writer Bengali from New Delhi.
MORE WORLD NEWS