It's a vote that could alter - or, opponents say, endanger - the democratic traditions of this key USA ally.
For "No" campaigners, though, the amendments dismantle democracy, a view shared by the Venice Commission, which advises the leaders of 61 member states, including Turkey, on rule of law. A narrow majority of Turks will vote "Yes", two opinion polls suggested on Thursday, putting his support at only a little over 51 per cent. Erdogan is predicting at least a 55 percent margin for "yes".
Erdogan said the proposed reforms could help counter a series of threats, including a failed military coup past year and a string of deadly bombings, some attributed to the Islamic State group. A "no" would mark a major setback for Recep Tayyip Erdogan, Turkey's president, though far from a fatal one.
In this atmosphere, referendum opponents say it's hard to run an effective campaign.
Turkish police detain demonstrators during a protest outside a university campus in Ankara against the dismissal of academics from universities following a post-coup emergency decree, February 10, 2017.
The president says the new system will resemble those in France and the United States and will bring stability in a time of turmoil marked by a Kurdish insurgency, Islamist militancy and conflict in neighbouring Syria that has led to a huge refugee influx. The governments of Austria, the Netherlands and Germany even banned Turkish government officials from making pro-"yes" speeches to Turkish citizens in these countries.
The proposals also have drawn strong global condemnation, "A unsafe step backwards in the constitutional democratic tradition of Turkey", wrote the Venice Commission of the Council of Europe, adding, "The Venice Commission wishes to stress the dangers of degeneration of the proposed system toward an authoritarian and personal regime".
Under the existing constitution, Turkey's chief executive is the prime minister, chosen by the parliament.
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The president would appoint an unlimited number of vice-presidents.
Many Turks voting "Yes" want a powerful president to confront terrorism and the campaign against Islamic state in Syria.
The current setup requires the president to be nonpartisan.
Erdogan's supporters argue that a "yes" vote would enable the president to speed up policy implementation that's normally complicated by Parliament. Prime Minister Binali Yildirim insisted the ruling Justice and Development Party (AKP) was as dedicated to a unitary structure as the nationalists and vowed to resign if the changes led to such a system.
"Whatever the outcome, there won't be a respite from the growing authoritarianism" of Erdogan, said Wolfango Piccoli, the London-based co-president of political risk advisory Teneo Intelligence.
Erdogan argues that as Turkey's first president to be directly elected by the people - instead of the parliament - he has a wider mandate than previous presidents.
Mr Erdogan has been in power for 14 years, longer than any Turkish leader since the founder of the republic, Kemal Ataturk.
If passed, proposals like the abolition of high military courts would take effect immediately, with others enacted after elections scheduled for November 2019, unless they're called earlier. "If the president is not speaking out, if he acknowledges it, then what would be the decision of nationalists - who are against a federal structure and support a unitary one - be (in the referendum) in two days?"