The reclusive imam whose crumbling political marriage of convenience with Turkish Prime Minister Recep Tayyip Erdogan has threatened the stability of the West's biggest ally in a turbulent region lashed out Monday at his onetime partner, the strongest sign yet of an irreparable split.

In comments he made to The Wall Street Journal, Fethullah Gulen, a charismatic cleric who preaches a message of tolerance to his millions of followers from his self-imposed exile in Pennsylvania's Pocono Mountains, accused...