The change, he says, will bring much-needed stability to Turkey.
Opinion polls have given a narrow lead for a "Yes" vote, which would replace Turkey's parliamentary democracy with an all-powerful presidency and may see Erdogan in office until at least 2029.
Erdogan could formally rejoin the party he co-founded, the ruling Justice and Development Party (AKP).
Turkish President Tayyip Erdogan addresses his supporters during a rally for the upcoming referendum in Istanbul, Turkey, April 15, 2017.
Mr Erdogan's tenure as prime minister and president since 2003 has starkly polarised Turkey's hugely diverse society.
"The 18 articles foresee a very loose separation of powers", said Ahmet Kasim Han, an associate professor of worldwide relations at Kadir Has University in Istanbul.
(AP Photo/Burhan Ozbilici). A supporter of the "NO" campaign holding a poster of Turkey's founder Kemal Ataturk, dances with others during a gathering ahead of the Sunday referendum, in Ankara, Turkey, Friday, April 14, 2017.
"It's going to be a very close call, and both "yes" and "no" are equally probable as outcomes", Han said.
They also argue that the current constitution, written by the generals who ruled Turkey in the years following a 1980 coup, still bears the stamp of its military authors and, despite numerous revisions, must be overhauled.
Turkey referendum: What are the main issues and process?
But presidential decrees will not be permitted on issues concerning human rights or basic freedoms, or to override existing laws. Ibrahim Kaboglu is a leading professor in constitutional law - he's deeply concerned about the repercussions of the referendum.
Erdogan views the HDP as the political wing of the Kurdistan Workers Party (PKK) militant group, which has waged a three-decade insurgency in the southeast and claimed responsibility for a deadly attack on a police compound in Diyarbakir on Wednesday.
Mr Erdogan's drive for a "yes" vote has led him into deeper conflict with Europe, where there are large Turkish populations.
In Kulunk's office on Camlica Hill, once a historical hunting ground and now a popular viewing point, a signed picture of Erdogan hung on the wall next to portraits of Ataturk.
The "Yes" campaign also hit a last minute hitch when the Nationalist Movement Party (MHP), the AKP's partner in promoting the changes, reacted angrily to comments by a presidential adviser suggesting a federal system could be imposed in Turkey.
The vote comes at a perilous time.
Nine months ago, Erdogan beat off a military coup. Many believe he has improved services and health care, and given a voice to pious Muslims who at times felt marginalized by more secular governments. When ministers attempted to campaign in European Union countries, there was a clampdown on rallies and Erdogan responded by calling leaders "fascists" and "Nazis".
According to the United Nations rapporteurs, the right to education has been targeted by the government since the coup attempt as a significant proportion of the public servants who were dismissed worked as schoolteachers or for the Ministry of Education. "We have paid dearly for these delays".
Erdogan and government officials are accused of using state resources and official functions such as openings of infrastructure projects to campaign in favor of the changes. "No" campaigners say they have recorded more than 100 incidents of intimidation, beatings and arbitrary detentions.
Erdogan lashed out at the OSCE report in a campaign speech Friday in the central city of Konya. A "Yes" vote would put the country in danger, he said. "Everyone will know their place", he said. So they say they saw "yes" everywhere and did not see "no".