The rallies were largely peaceful even as Rouhani supporters faced off against smaller crowds supporting his main rival, hard-liner Ebrahim Raisi, though police rushed reinforcements to break up Rouhani rallies that grew large enough to block traffic.
A victory for Rouhani could lead to a further loosening of limits on personal freedom, while a hard-line win could set Iran up for a renewed bout of confrontation with the West at a time when U.S. President Donald Trump has called for a tougher line on Iran.
In March, in an Iranian New Year speech, he announced the next 12 months would be the "Year of Resistance Economy" and in a video the same month he offered a thinly-disguised critique of the Rouhani administration, saying, "I am aware of the people's situation; therefore, I can sense the bitterness of the people, especially the lower classes, from economic problems such as inflation, unemployment, discrimination, equality and social harms". Rouhani failed to keep his promises.
Raisi, who is seen to be very close to Khamenei, had previously attacked Rouhani for failing to deliver promised improvements to the economy.
Raisi is a mid-ranking figure in the hierarchy of Iran's Shi'ite Muslim clergy but has been a senior official for decades in the judiciary which enforces clerical control of the country. While a Raisi win "could usher in a more confrontational approach to domestic and regional policies", she warns.
"Raisi is the true face of the Islamic Republic, while Rouhani is a façade", wrote Abrams.
"[With a heavy turnout], regardless of who gets the most votes, the real victor would be the people and the Islamic Republic system", he said. The day the regime falls will have been brought that much closer. "We will not retreat", Rouhani said in a campaign rally. Rouhani's signature accomplishment has been his nuclear deal, which could be in jeopardy if he loses power, even though it was officially endorsed by Khamenei and all candidates say they will abide by it. He is appealing to a significant segment of the country's population that is eager for political change in the country and rapprochement with the wider world. Moreover, given his record, Raisi is likely to support ballistic missile tests and arms shipments to Syrian President Bashar al-Assad, moves that "undoubtedly provoke a strong response from Washington", according to RBC.More news: Trump administration triggers launch of NAFTA renegotiations
More news: Microsoft slams spy agencies for hoarding hacking tools used in cyber attacks
More news: Trump and Netanyahu secretly spoke after Israel leak report
For that reason, perhaps, Rouhani has seemed at odds with elements of the regime in recent years. If he wins, the backlash from more conservative forces may be swift.
"A sanctions snap-back could not only deter foreign investment in the Iranian energy sector but could also curtail the country's ability to sell its barrels overseas", they added. "Why? Because they know that these are the areas which the reformists and the moderates want to improve".
The reformists and the principlists are on two opposite sides of Iran's political spectrum, the reformists lean to the left and the principlists to the right. "Therefore they will try to weaken them and make them look feeble in front of their voters by doing things which the winning camp has stood against".
"Reformist President Mohammad Khatami won in 1997 and 2001 with 79.9 percent and 66.8 percent voter turnout, respectively".
Supporters of Iranian President and candidate elections Hassan Rouhani distribute brochures ahead of the Iranian presidential election in the streets of the capital Tehran on May 17, 2017.
Iran's seventh president, elected in 2013, Mr Rouhani succeeded in securing agreement on Iran's nuclear programme, the partial lifting of sanctions, and restoration of relations with the West. But these trends are apparently altogether separate from the more tepid trends within the reformist wing of Iranian politics.