(CNN) One of Iraq's leading Shiite clerics and powerful politicians has called on other parties to back him in removing Prime Minister Adil Abdul Mahdi after weeks of protests.

Muqtada al-Sadr , who leads the largest political block in parliament, invited Iraq's second largest party on Tuesday to cooperate with him in "immediately" holding a vote of no confidence in Mahdi.

In a letter addressed to the prime minister, al-Sadr said Tuesday that "I thought asking you to call for early elections will preserve your dignity. But, if you refuse, I ask Mr. Hadi Al-Amiri to cooperate in the withdrawal of confidence from you immediately, to work together in changing the Electoral Commission and its law, and agree on comprehensive reforms, including the provisions of the constitution to be put to the vote and in the absence of the Parliament vote, then the people will choose."

Al-Amiri, who is the leader of the Iran-backed Badr Organization, accepted al-Sadr's invitation on Tuesday, saying "we will work together to achieve the interests of the Iraqi people and save the country as required by the public interest."

Earlier this week, al-Sadr had called on Mahdi to come to parliament and announce "early elections under the supervision of the United Nations," which the prime minister refused, citing constitutional and operational challenges.

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