In Friday's debate, Tehran's mayor and presidential candidate Mohammad Bagher Qalibaf said that Rouhani and candidate Eshaq Jahangiri received "highly subsidized" properties from the government.
"The biggest fear of the conservatives was that Qalibaf may outperform Raisi on Friday, but not be able to pose a serious challenge, let alone beat Rouhani in the run-off", said Hossein Rassam, a former adviser to Britain's Foreign Office.
"The move comes with a view to preserving the revolution's front and its values", he added.
The election is seen largely as a referendum on Rouhani's outreach to the rest of the world following the country's landmark 2015 nuclear accord with world powers.
All Iranian presidents have won re-election since 1981 and Rouhani remains the favourite.
Although the President has long cast himself as an insider and pragmatist rather than a gung-ho reformer, he seems to have shed that moderate image in recent days, seeking to energise voters who want less confrontation overseas and more freedom at home. While the nuclear deal allowed Iran to resume crucial oil exports to Europe and sign billion-dollar airplane deals, chronic unemployment and inflation remain major concerns.
Iran's presidential election may turn on turnout.
Raisi, a 56-year-old Shiite cleric, has close ties to Iran's supreme leader, Ayatollah Ali Khamenei, who last year appointed him as head of Iran's largest charitable foundation, the Astan Quds Razavi.More news: Celtics blow out Wizards 123-101
Both Qalibaf and Raisi are conservative rivals of incumbent President Hassan Rouhani who hopes not to lose his current sovereignty.
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.
Rouhani said: "Mr Raisi, you can slander me as much you wish. We have resolved some problems and bigger problems remain for us to resolve on this hard path with him".
A picture taken on April 13, 2017 shows former Iranian minister Mostafa Hashemitaba registering his candidacy for the upcoming presidential elections at the Interior Ministry in the capital Tehran. Raisi has been campaigning on that, proposing cash payments for the poor that proved popular in the past under Ahmadinejad.
But when I go out on the street here and ask several people who they will vote for, a lot of them are still saying Hassan Rouhani because he is a man promising to change this country.
He said he was "aware" of "the misery" that the so-called Kurdish kolbars, porters carrying goods across the Iran-Kurdistan Region borders, have faced and promised to ensure that border revenues are spent to revive the economy in the region.
High voter turnout has generally favored moderate and reformist candidates in Iran.
During the previous polls in June 2013, President Rouhani garnered 50.7 percent of a total of over 36 million votes.