Karroubi asked all Iranians to take part in the election and to protect the democratic process by choosing their preferred candidate, but said: "I will vote for Rouhani of course".
The coalition between the two conservative nominees will benefit the country's economy in a post-election era, if Raeisi manages to assume power through the election.
Rouhani's hardliner competitors, Tehran's mayor, Mohammad Bagher Qalibaf, and Ebrahim Raisi, chairman of Astan Quds Razavi, one of the Muslim world's wealthiest charities, have concentrated their campaign on the economic problems, which Iranians have continued to face since the deal.
Incumbent President Hassan Rouhani backed by reformists is seeking re-election in the upcoming election.
He was left with no option but to quit the race, however, when the main conservative parties and clerical bodies threw their support behind Raisi, a jurist and Shi'ite cleric who studied at the feet of Supreme Leader Ayatollah Ali Khamenei. It is likely others will drop out to solidify support for other candidates, especially as one of them is serving as vice president in Rouhani's government. Mansoura Arfanian, an activist in the campaign of the conservative Ebrahim Raisi, told KUNA that the campaign has intensified publicity to lure more young citizens of diverse intellectual tendencies.
The former prosecutor is now head of a multi-billion-dollar charitable foundation that manages donations to Iran's holiest shrine in the city of Mashhad. It is increasingly hard for Rouhani to sell the deal to voters now given Trump's rhetoric.
North Korea reports on South's presidential election
Moon Jae-in emerged the victor in the May 9 presidential election to have collected 41.1% of the votes. The two sides agreed on the importance of denuclearizing the peninsula.
The nuclear deal between Iran and the six world powers ended a decadeslong crisis that removed sanctions on almost 80 million Iranians and potentially averted another military confrontation in the Middle East involving the United States.
The two candidates were the top conservative challengers to President Hassan Rouhani, a pragmatist whose government negotiated a 2015 deal with world powers to rein in Iran's nuclear program in exchange for lifting worldwide sanctions. He called a vote for Raisi a "crucial decision" to "preserve the unity" of the revolution. As many as 16.2 percent of the respondents said they had not yet decided whether to vote or not.
The Iranian Interior Ministry on April 20 announced the final list of six candidates vetted by the Guardian Council to run for president.
He lost to Rouhani during the presidential election of 2013. Raisi has been campaigning on that, proposing cash payments for the poor that proved popular in the past under Ahmadinejad.
That past has anxious moderates and reformists in Iran.