NAJAF, Iraq — No one man in Iraq has more power to change the outcome of the country’s elections on Sunday than a frail cleric who lives in an ascetic house in this holy city. And yet he has refused to wield it, shaping the relationship between Islam and the state at a crucial juncture in Iraq’s history.

The cleric, Grand Ayatollah Ali al-Sistani, Iraq’s senior Shiite spiritual leader, has repeatedly refused to endorse any of the electoral coalitions fighting for votes among the country’s Shiite majority. He did so most recently three days ago.

For a figure whose electoral blessing would be decisive, it reflects a reversal from the role he played in orchestrating a unified Shiite coalition during Iraq’s first national elections in 2005.

He has urged Iraqis to vote — implored them, in fact, in edicts that carry the weight of religious law — but insisted on maintaining the neutrality of the Shiite religious elite, known as the marjaiya, that he and three other senior clerics now represent.