NAJAF, Iraq — As the top spiritual leader in the Shiite Muslim world, Grand Ayatollah Ali al-Sistani has instructed his followers on what to eat and how to wash, how to marry and to bury their dead. As a temporal guide, he has championed Iraqi democracy, insisting on direct elections from the earliest days of the occupation, and warned against Iranian-style clerical rule.

Frail at 81, he still greets visitors each morning at his home on a narrow and sooty side street here, only steps from the glimmering gold dome of the Imam Ali Shrine. But the jockeying to succeed him has quietly begun, and Iran is positioning its own candidate for the post, a hard-line cleric who would give Tehran a direct line of influence over the Iraqi people, heightening fears that Iran’s long-term goal is to transplant its Islamic Revolution to Iraq.