In the wake of the U.S. withdrawal from Iraq, Iran is stepping up plans to dominate its western neighbor and longtime adversary, and a key project is to ensure the holy city of Najaf in Iraq doesn't reclaim its status as the spiritual center of Shiite Islam.

Call it the battle of the ayatollahs.

The Iranian effort has been given greater urgency by the threat against Tehran's key Arab ally, the Syrian regime of President Bashar Assad.

Syria is Iran's gateway to the Levant and the Mediterranean, a passage vital to Tehran's strategic aim of expanding Shiite dominance into the largely Sunni Arab world.

The prospect that Iraqi Prime Minister Nouri al-Maliki, a Shiite, could be unseated in a burgeoning rift with Sunnis, rival Shiites and Kurds has galvanized Tehran in recent days to take steps to ensure that no such thing occurs and that Maliki stays in power -- under Tehran's thumb.

But there's an even more important and probably far-reaching Iranian effort under way by the Tehran leadership and Iran's powerful Shiite clergy: ensuring that the holy city of Qom south of Tehran is the religious center for the world's 200 million Shiite Muslims, rather than Najaf.

For centuries, Najaf was the holiest city of Shiite Islam, the site of the tomb of Ali, the Prophet Mohammed's cousin and son-in-law who is revered by Shiites as Mohammed's rightful successor following his death in 632 rather than the one chosen by the rival Sunnis.

Najaf lost its pre-eminence during Saddam Hussein's suppression of the Shiite faith and the emergence of fundamentalist Iran in 1979 as the beacon of radical Islam.

But with the fall of Saddam and his Sunni-led regime, Najaf has undergone a renaissance as the Vatican of the Shiite faith. It's playing a critical role in Iraq's rebirth.

This theological rivalry is particularly potent right now because the cleric who has held the position of al-marjaa al-akbar, "the greatest object of emulation," of the world's Shiites, Ayatollah Ali Sistani, has his seat in Najaf. He's 81 and could step down soon.

The reclusive Sistani is Iranian by birth but he's the acknowledged leader of Iraq's Shiites and deeply opposes the concept of velayat-e faqih, or clerical rule, that Ayatollah Ruhollah Khomeini established in 1979.

Iran is promoting a conservative cleric close to the supreme leader, Ayatollah Ali Khamenei, as Sistani's successor, a move that would give the Tehran leadership an immensely powerful platform to influence Iraqi Shiites, the majority sect, and, as many Iraqis fear, transplant Iran's Islamic revolution in Iraq 24 years after the two countries' crippling 1980-88 war.

After the 2003 U.S. invasion, Sistani, a member of the "quietist" school of Shiite Islam, played a key role in shaping post-Saddam Iraq and won the ear of the Americans.

The man Tehran wants to replace him, Grand Ayatollah Mahmoud Hashemi Shahroudi, 63, is a prominent member of Iran's ruling clerical hierarchy. He played a major role in crushing Iran's reformist movement in the 1990s.

He's a former chief of Iran's judiciary, with a brutal record against opponents of the regime, and has intimate connections with Khamenei's hard-line conservative cadre.

Tehran is financing the Iraq-born Shahroudi and for months his emissaries have been building a support network across Iraq. He opened an office in Najaf in October.

"He's there to prepare himself for after Sistani," observed Iranian-born analyst Mehdi Khalaji of the Washington Institute for Near East Policy.

Maliki recently visited Tehran and met with Shahroudi, who's spiritual mentor of the premier's Ad-Dawa party.

Historian Reidar Visser, who specializes in Middle Eastern affairs, noted in his political blog this "did nothing to kill the rumors about some kind of Iranian design on the holiest center of Iraqi Shiism."

The frail Sistani, one of only five living grand ayatollahs, has long been secluded in Najaf and hasn't left his home since 2004.

Despite the reverence in which he's held, young Iraqi Shiites say Sistani's out of touch with modern life and favor more dynamic spiritual leaders.

The Iranians are exploiting this for all its worth.

The stakes are high. If Sistani succeeds in reestablishing Najaf as the center of gravity of global Shiism, marginalizing Qom, Iran's authority in Iraq, along with the concept of clerical rule, will be severely reduced.