Nearly a quarter of Iran's parliamentary seats are at stake Friday in an election in which reformists want to consolidate their recent comeback and minimise the clout of hardline lawmakers.
The second round run-offs were triggered because no candidate in 68 constituencies managed to win 25 percent of votes cast in the initial nationwide ballot on February 26.
Reformists who backed the country's moderate President Hassan Rouhani made big gains in the first round following Iran's implementation of a nuclear deal with world powers, which lifted sanctions blamed for long hobbling the economy.
Conservative MPs, including vehement opponents of the West who openly criticised the landmark agreement that reined in Iran's atomic programme, lost dozens of seats.
In the capital, even the head of the conservative coalition was ejected, in a wipeout that saw the reformist faction -- known as the List of Hope -- win all 30 seats.
Despite that loss, the conservatives held on nationally, winning 103 seats against 95 for their reformist and moderate rivals in the 290-member parliament.
But the split result -- other seats were won by nominally independent candidates and minorities -- meant no faction won a majority.
Friday's contest, which sees the top two candidates in initial voting stand for the remaining seats in cities such as Tabriz and Shiraz, as well as dozens of smaller towns, could change that. A run-off in the city of Isfahan has been postponed to a later date.
Mohammad Reza Aref, leader of the pro-Rouhani List of Hope, has set a target of at least another 40 seats, reiterating that February's poll showed Iranians want change and greater influence in parliament would make reforms more likely.
Unlike the vote two months ago, there has been little attention on Friday's election, which has been overshadowed by concern in Tehran that no benefits are being generated by the nuclear deal.
The agreement between Iran and six powers -- Britain, China, France, Russia and the United States, plus Germany -- became possible because supreme leader Ayatollah Ali Khamenei backed Rouhani's government.
However Khamenei and top officials have since warned that the United States is not keeping its side of the bargain and is instead erecting barriers for European banks seeking to work in Iran.
February's voting came less than six weeks after the nuclear deal formally came into force.
Although Rouhani's reformist and moderate allies didn't win the most seats, the president will likely be able to muster support from key backers across Iran's political spectrum and create a working majority when the new parliament is sworn in next month.
The election gave him more leverage to push for reform-minded legislation, and economic changes, including new laws to encourage foreign investment and privatisation of state industries, are expected.
The most dramatic outcome in February's election, however, was the resurgence of the reformists, a political camp largely silenced after a disputed election in 2009 saw Rouhani's hardline predecessor, Mahmoud Ahmadinejad, returned to office.
That vote was followed by bloody street protests in which dozens of people were killed.
If conservatives perform more strongly on Friday amid concern over the nuclear deal -- Rouhani's main achievement since his landslide election win in June 2013 -- his hopes for a more pro-government parliament could founder.
The conservatives have not changed tack since February's voting, keeping up pressure over what they say is a silent agenda among reformists to give up the principles of the 1979 Islamic revolution.
"We hope that people in this round can have a parliament in line with the goals of Imam and the leadership by electing principlists," said Gholam-Ali Hadad Adel, head of the conservative coalition.
He was referring to revolutionary leader Ayatollah Ruhollah Khomeini and his successor Khamenei, who is Iran's ultimate authority.
From a population of almost 80 million -- 60 percent of whom are aged 30 or under -- 62 percent of 55 million eligible voters cast ballots in February. Around 17 million people are eligible to vote on Friday.