Karroubi told supporters that dissatisfaction with the current regime of President Mahmoud Ahmadinejad would bring the Reformist camp to power once again, Iran's Press TV reports.
"Public displeasure (with government policies) led to the victory of Khatami as a Reformist figure in 1997," he said. "Iran is experiencing the same conditions now, so public discontentment would pave the way for a high turnout in the upcoming election."
Massive turnout in the 1997 election led to Khatami taking 70 percent of the vote.
Karroubi, the former speaker of the Iranian Parliament, also called on his opponents to avoid "character assassination" during the campaign season, saying it would leave a stain on the election process.
Karroubi and former Prime Minister Mir-Hossein Mousavi are campaigning for the June 7 presidential contest on the Reformist ticket. Karroubi has so far discounted appeals to step aside in favor of Mousavi.
Ahmadinejad is expected to seek another term under the Principlist ticket.