Tehran mayor and a candidate in Iran's 12th presidential election, Mohammad Baqer Qalibaf, issued a statement on May 15 announcing that he is to withdraw from the presidential race while backing Ebrahim Raeisi.
Centrist-leaning President Hassan Rouhani faces an uphill battle for a second term.
"To protect this great ideal, I ask all of my supporters across the country to offer all of their support to the success of our dear brother Ebrahim Raisi".
On Monday, Tasnim news agency quoted a source from the conservative coalition as saying that Qalibaf's electoral campaign would continue to work in support of Raisi until the end of the elections.
It's not a given that most Qalibaf supporters will automatically switch to Raisi on May 19, said Aniseh Bassiri Tabrizi, research fellow at the Royal United Services Institute in London.
There was no immediate reaction from Rouhani.
He has accused incumbent President Hassan Rohani of mismanagement and favoring rich Iranians at the expense of the poor. With Qalibaf dropping out, there are now five candidates contesting, though others may choose to drop out in the coming days.
The change of tone, some experts say, is reminiscent of Rouhani's 2013 presidential campaign strategy.
"However, I don't think there will be a significant impact as Qalibaf got (just) six million votes in 2013". In 2003, a magnitude 6.6. temblor flattened the historic city of Bam, killing 26,000 people. "For a better future and to strengthen hopes, give your vote to Rouhani".
Iran's elections are similar to the French system in which a second round run-off is held between the top two candidates unless one wins 50 per cent or more in the first round.
The landmark nuclear deal struck in Vienna between Iran and a coalition of Western powers in 2015 aimed to reduce the global nuclear threat posed by Iran. Raisi, who is also running on a campaign to improve the economy, said in the Golestan province: "Citizen's right means the right of an unemployed to find job".
Raisi, a former attorney general, serves as the head of the Imam Reza charity foundation, which manages a vast conglomerate of businesses and endowments in Iran.
In the absence of credible polling in Iran, it is hard to gauge Raisi's popularity across Iran, particularly given that he has not run for presidency before. In the summer of 1988, he was one of the four sharia judges who ordered the mass execution of leftists and dissidents.
High voter turnout has generally favored moderate and reformist candidates in Iran.
Meanwhile, Iranian opposition figure Mehdi Karroubi, a 80-year-old who has been under house arrest since 2011, has said he would back Rouhani in the election.