The Turkish Parliament began debating legislation on Monday that would effectively rewrite the Constitution, vastly expanding the powers of Turkey’s president, Recep Tayyip Erdogan, and possibly allowing him to remain in power until 2029.

Mr. Erdogan, the dominant figure in Turkish politics for 14 years, was prime minister from 2003 until 2014. That year, unable to seek another term in that office, he ran for president, vowing to strengthen the powers of what had largely been a ceremonial job. He won easily.

The most dominant figure in Turkish politics since Ataturk, who created a modern secular republic out of the ruins of the Ottoman Empire, Mr. Erdogan has largely been able to exert his will. Since a failed coup attempt in July, he has used emergency powers to crack down on his opponents, suspending or dismissing tens of thousands of civil servants, police officers, academics and others. At least 260 people died in the coup attempt.

Mr. Erdogan’s ruling Justice and Development Party, known by its Turkish initials A.K.P., and the Nationalist Party, which is nominally in opposition but supports the constitutional proposal, are believed to have more than enough votes — at least 330 of 550 in the unicameral legislature — to pass the changes. The package would then be submitted to a national referendum in the spring. The Constitution has not been amended since 2010.