ISTANBUL — For more than three decades, Rachid al-Ghannouchi has preached that pluralism, democracy and Islam are harmonious. As his country, Tunisia, heads toward the first elections after the Arab revolts on Sunday, Mr. Ghannouchi, a renowned Islamic thinker, faces a reckoning between his principles and intentions.

Mr. Ghannouchi boldly predicted Wednesday that his Renaissance Party would win a majority in the elections, contested by more than 80 parties, to choose an assembly charged with drafting a constitution for a country that was once one of the Arab world’s most repressive. That would be one of the most startling achievements for an Islamist party in the Arab world since 1992, when the military in Algeria deprived a religious party there of an almost certain electoral victory, igniting a civil war.