Iran's President Hassan Rouhani registered on Friday to run for a second four-year term in the country's May election.

A pragmatist who engineered the country's landmark 2015 nuclear deal with world powers, Rouhani won the 2013 election by a landslide on a platform of ending the Islamic Republic's diplomatic isolation and creating a freer society.

"Once again, I am here for Iran, for Islam, for freedom, and for more stability in this country. I am urging all Iranians to vote for Iran and for Islam," Rouhani told reporters on Friday.

READ MORE: Will Iranians re-elect Rouhani?

Critics of the nuclear deal say that the supposed economic benefits of lifted sanctions have yet to trickle down to average Iranians, and palpable frustration in the streets has created a potential opening for conservative opponents.

Rouhani has vowed to remain loyal to the nuclear deal.

"From now on, protecting the deal is one of the most important economic and political issues," he said.

Influential Shia leader Ebrahim Raisi, an ally of Supreme Leader Ayatollah Ali Khamenei, appears to be the leading hardline candidate so far. He also registered to run on Friday.

But despite months of talks, hardliners have been unable to unite behind a single candidate, and Khamenei has not yet intervened to make them do so.

Within Iran's complex mix of clerical rulers and elected officials, Khamenei has the final say on all state matters.

Though Rouhani won in a single round with more than 50 percent of the vote in 2013 - an election in which no other candidate took more than 17 percent - he could face a tougher campaign this time if hardliners join forces against him.

The five-day registration period for the May 19 election began on Tuesday and will be followed by a process of vetting of the hopefuls by a hardline watchdog body, the Guardian Council.

The Guardian Council, which normally does not approve dissidents or women, will announce the final list of candidates by April 27.

More than 950 people have signed up so far for the vote, among them the hardline former Iranian president, Mahmoud Ahmadinejad, who registered on Wednesday.