The 73-year-old cleric has ruled Iran since the death of the republic's stern founder, Ayatollah Ruhollah Khomeini, in 1989 and has crippled the outgoing two-term president, Mahmoud Ahmadinejad, for daring to challenge the grip of the clerical regime Khomeini established.
Iranian presidents are subservient to the supreme leader but Ahmadinejad battled the clerical leadership for more political power -- and lost.
He was constitutionally barred from running for a third consecutive term but had sought to ensure he would be succeeded by his chief of staff, Esfandiar Rahim Meshaie, whose son is married to one of Ahmadinejad's daughters and who many say exercised a Svengali-like influence over the outgoing president.
Ahmadinejad's political career is clearly over. So is Meshaei's. The establishment has branded him a "sorcerer," a spy and a devil worshipper. The word is that he "sleeps with a gun under his pillow." Some of his associates have been arrested in recent weeks.
Khamenei has made sure that whoever succeeds Ahmadinejad will remain subservient to the supreme leader and the mullahs.
So whoever wins, even the lone "reformist" that made it through Khamenei's closed-door selection process, along with a posse of Khamenei-approved candidates, there will be no repeat of Ahmadinejad's ill-fated challenge.
But the question that runs subliminally through this election, although it may have got swamped in Khamenei's power play, is: How long will clerical rule -- velayat-e faqih, or "rule by the jurisprudent" as laid down by Khomeini -- go on in Iran as the economy, frazzled by Western sanctions, sinks and the Shiite mullahs face a looming showdown with Islam's mainstream Sunni sect to resolve a 1,300-year-old religious schism?
On Friday, much will depend on the turnout. A large turnout, 80 percent or more, will be seen as legitimizing the existing power structure. But after the upheaval of the 2009 election, when Khamenei made sure Ahmadinejad won re-election to the anger and dismay of much of the electorate, nobody's expecting a mass turnout.
The harsh crackdown by the regime in 2009, in which more than 100 people were killed and two leading reformists remain under arrest, has disillusioned many Iranians about elections.
The regime's biggest fear is that voters will stay at home this time around, and for weeks it's been bombarding the electorate with patriotic slogans to get out there and vote.
The selection of the candidates for Friday's poll, overseen by Khamenei and the 12-member Council of Guardians, which must approve all candidates, purged some 700 applicants.
These included former president and Khomeini lieutenant Ali Akbar Hashemi Rafsanjani, who might have given the establishment a run for its money, and Meshaei.
The eight men who are competing in the republic's 11th presidential election since it was founded in 1979 after Khomeini toppled Shah Mohammad Reza Pahlavi, are all closely associated with the government in one way or another.
Khamenei's leaving little to chance. Many observers say Iran's moving toward greater clerical control, backed by the Islamic revolutionary Guard Corps, the powerful, ideologically sound elite force created by Khomeini as the clerical regime's praetorian guard.
The eight include former Foreign Minister Ali Akbar Velayati, Khamenei's top international affairs adviser; former Revolutionary Guards commander Mohsen Rezaie; Saeed Jalili, anti-Western head of Iran's Supreme National Security Council and its chief nuclear negotiator; and former Tehran Mayor Mohammad Baqir Qalibaf, ex-commander of the Guard Corps' air force.
Khamenei hasn't indicated whether he has a favorite but most see Jalili, another former Guards officer, as the man to watch.
The only moderate is Hassan Rowhani, a centrist ally of Rafsanjani, nuclear negotiator in 2003-05 and a champion of rolling back the U.S.-led sanctions.
He's been endorsed by the reformist camp headed by former President Mohammed Khatami, whose people were crushed in the 2009 election and the nationwide crackdown that followed.
These days, an endorsement like that's pretty much a kiss of death, unless reformist voters turn out in extraordinary numbers Friday.
It's widely expected there will no clear-cut winner, meaning a runoff June 21 between the two top vote getters.
Senate Democrats to pull all-nighter on climate change
Scarlett Johansson steps out with fiance after pregnancy reveal