Members of the Reformist Association of Combatant Clerics called for all members of the party, including all presidential candidates, to unite and coalesce on their opinions, the Iranian Press TV reports.
Khatami, a founding member of the party, withdrew from the Iranian presidential race last week, saying he did not want to dilute the Reformist candidate pool following a decision by former Prime Minister Mir-Hossein Mousavi to re-enter the race as a Reformist.
Iranian analysts have speculated Khatami stepped down amid political pressure and veiled threats, however.
The former speaker of the Majlis, the Iranian parliament, also announced his candidacy for the June election as a Reformist, though calls have mounted for him to step aside to make room for Mousavi.
The statement from the Association of Combatant Clerics said unity, and not political disputes, are needed in the run-up to the presidential contest.
It is believed incumbent President Mahmoud Ahmadinejad will seek another term in office in the June 12 election.