Raisi's election would be seen as a rebuke of the nuclear deal and, as Suzanne Maloney of the Brookings Institution told the Huffington Post, while the deal wouldn't fall apart immediately, it would "erode nearly inevitably as a result of the lack of commitment from Iran" under Raisi.
Voting in Iran's presidential election was extended twice on Friday due to long lines at polling places where voters are selecting the Islamic republic's next president. Analysts have suggested a high turnout will aid Rouhani in securing a second four-year term.
Rouhani has decisively defeated Khamenei's protege, hardline judge Ebrahim Raisi. Around 56 million were eligible to do so.
Iran's interior minister Abdolreza Rahmani-Fazli has said the results of the country's upcoming presidential election will be announced all at once on Friday, and not at different stages. It is the first presidential vote since the Iran nuclear agreement was reached with world powers in 2015.
"The wide mobilisation of the hardline groups and the real prospect of Raisi winning scared many people into coming out to vote", said Nasser, a 52-year-old journalist.
Now, having stoked his supporters' yearning for change, he faces the harder task of satisfying them without bringing a backlash from the conservatives who still control most of the levers of power.
For ordinary Iranians, the election presents a stark choice between competing visions of the country.
With results from urban areas still to be counted - and seen as more likely to favour Rouhani - the initial figures pointed towards a landslide for the incumbent.
Every incumbent president has been re-elected in Iran since 1985, when Ayatollah Khamenei himself won a second term.
Mostafa Aqa-Mirsalim and Mostafa Hashemi-Taba, the other two candidates, so far received 455,211 and 210,597 votes respectively.
Rouhani was elected on the promise of creating a more open society. He is subordinate to the supreme leader, who is chosen by a clerical panel and has the ultimate say over all matters of state. The Revolutionary Guards are part of Iran's Armed Forces and entrusted with the task of protecting the Islamic nature of the country.
Jonah Hull of Al Jazeerareported from Tehran that large crowd numbers at the polls "would tend to favor the moderate reformist side of President Rouhani, and anything less than 60% would spell trouble for him". The elections are also for city and village councils.
He pushed boundaries during the campaign, criticising the continued arrest of reformist leaders and activists, and calling on security agencies to not interfere in the vote.
Supporters of the two leading candidates honked, blared music and held pictures of the hopefuls out of vehicle windows on the traffic-clogged and heavily policed streets of Tehran late into the night Thursday, ignoring a ban on campaigning in the final 24 hours before the vote.
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