By Serena Chaudhry and Raheem Salman

NAJAF, Iraq (Reuters) - Cleric Moqtada al-Sadr, head of a powerful Shi'ite movement in Iraq, on Sunday called for more political reforms, saying he would back a no-confidence vote against the prime minister if they were not made.

Sadr, a Shi'ite cleric who led uprisings against the U.S. presence before American forces withdrew last December, is now an influential player in government after his bloc's support of Iraqi Prime Minister Nuri al-Maliki helped secure his position.

"Our main demand and the last demand is reforms," Sadr told journalists during a rare news conference at his family home in the Shi'ite holy city of Najaf, southern Iraq.

"I said and I am still saying that there is a promise from me to the other blocs if the votes (for a no-confidence motion) reach 124, my 40 votes are with them."

He did not elaborate on what kind of political reforms he would like to see and said he would only support a no-confidence vote provided it did not prove harmful to Iraqis.

Iraq's main Shi'ite, Sunni and Kurdish factions have been locked in a dispute over power since U.S. forces left.

Maliki's opponents have been calling for a vote of no- confidence against the Shi'ite leader, but have so far failed to muster enough support for the motion.

The ruling National Alliance was formed when Maliki's party linked with Sadrists and other Shi'ite groups.

A successful ballot would be the most serious challenge to Maliki in his six years in office, potentially sinking the government and escalating sectarian tensions in a country still pulling back from years of war.

OLD DIFFERENCES

Maliki and Sadr, once foes when the populist cleric's militia battled U.S. and Iraqi security forces, united in 2010 after nine months of political wrangling following an inconclusive vote. Continued...