A heated debate about who will be allowed to run in Iran's presidential election has erupted five months before the vote, stoking concerns about a repeat of the protests that followed the contested 2009 poll.

At the heart of the controversy is whether the vote will be what critics of Iran's electoral system call "free" — that is, cast with a ballot that includes candidates from all of Iran's various political factions and not just so-called principlists, the conservatives who are loyal to the Shiite Muslim clerical establishment that rules Iran.

The loudest calls for an open field of participants are coming from two former presidents and the outgoing one, Mahmoud Ahmadinejad. They are trying to ensure that their political allies are not barred from running by the Guardian Council, the powerful committee of clerics and jurists that vets the eligibility of potential candidates. Half of the 12-member council is appointed by Iran's supreme leader, Ayatollah Ali Khamenei, and half by Parliament from nominees who are beholden to the supreme leader.