If there was a global contest for winning elections, Turkey's President Recep Tayyip Erdogan would see himself as the undisputed -- and undefeated -- heavyweight champion of the world.
In one-and-a-half decades since his ruling party came to power, Erdogan has taken part in 11 elections -- five legislative polls, two referenda, three local elections and a presidential vote -- and won them all.
On Sunday, Erdogan faces his twelfth and arguably biggest ballot box challenge since his ruling Justice and Development Party (AKP) came to power in 2002, in a referendum on expanding his powers.
His supporters see the new system as a historic change that will create efficient government. But for detractors, it is a dangerous step towards one-man rule in the NATO member and EU candidate state.
Fighting for votes in every corner of the country, Erdogan has kept up a punishing schedule of daily rallies seeking to woo doubters with his indefatigable campaigning.
Prowling around the stage with a wireless microphone like a rock star, Erdogan bellows at the crowds: "Do you want a strong Turkey?".
Known to his inner circle as "beyefendi" (sir) and to admirers as "reis" (the chief), Erdogan is supreme on stage, holding the audience in the palm of his hand with near-matchless public speaking skills.
- 'Balancing act' -
Yet while Erdogan is seen in Western media as a near omnipotent sultan, there are constraints to his rule, according to Asli Aydintasbas, senior fellow at the European Council on Foreign Relations.
"Erdogan has to continue to win votes in order to stay in power and campaign round the clock," she said.
In order to win the referendum Erdogan has to perform a "delicate balancing act" of winning votes from both Kurds and nationalists, she added.
Erdogan also comes to the referendum after the most turbulent year of his political life which saw a slew of terror attacks, worsening relations with Europe and above all the July 15 failed coup.
He appeared on the FaceTime app on live TV to urge supporters to flood streets and defeat the coup, saying he escaped being killed by just 15 minutes before returning in triumph to Istanbul.
The president has courted ever more controversy as authorities jailed over 47,000 under a state of emergency which has lasted nine months so far.
There has even been talk of fissures within the ruling AKP and with his two other party co-founders -- former president Abdullah Gul and ex deputy prime minister Bulent Arinc -- both deafening in their silence by failing to endorse the new system.
- 'My crazy projects' -
If the new constitution is passed, Erdogan could stay in power until 2029, by which time the energetic president, 63, would be aged 75.
Erdogan appears determined to leave a legacy at least as significant as Turkey's modern founder Mustafa Kemal Ataturk whose picture hangs next to his at rallies.
He has embarked on a hugely ambitious drive to modernise Turkey's infrastructure with a new bridge and two tunnels spanning the Bosphorus, high speed trains and the construction of a third airport for Istanbul, schemes he affectionately refers to as "my crazy projects".
But critics worrying of a creeping Islamisation of Turkey's officially secular society with a surge in mosque building, use of Islamic schools and the abolition of all restrictions on the headscarf in public life.
Born in Istanbul but brought up by the Black Sea, Erdogan is intensely proud of the humble origins from which he rose to be Turkey's most powerful politician since Ataturk.
He gained prominence in the nascent Islamic political movements that were starting to challenge secular domination, becoming a popular mayor of Istanbul in 1994.
He was jailed for four months for inciting religious hatred when he recited an Islamist poem, a term which only magnified his profile.
Founding the AKP after the previous Islamic party led by his mentor Necmettin Erbakan was banned, Erdogan spearheaded its 2002 landslide election victory and became premier less than six months later.
It was in these early days that the AKP, lacking allies, forged an alliance with the movement of US-based preacher Fethullah Gulen that would end with the sides becoming sworn enemies and Gulen blamed for masterminding the coup bid.
- Return to pragmatism? -
Protests in 2013 over plans to build a shopping mall on an Istanbul park provided a rallying cause for secular Turks but Erdogan came out fighting, famously slamming the protesters as "capulcu" ("hooligans").
In 2014 Erdogan was elected president in the first ever popular vote for the post and moved into a vast new presidential palace opponents denounced as a needless extravagance.
In June 2015 elections the AKP won the most votes but lost its overall majority for the first time. But Erdogan swatted away any proposal of a coalition and called new elections in November where the majority was restored.
Whatever the April 16 referendum's outcome, all eyes on April 17 will be turned to whether Erdogan softens the campaign rhetoric and adopts a more conciliatory stance, especially on the EU membership bid and the shattered peace process with Kurdish militants.
"He has been extremely pragmatic in the past, often when you least expect it," said Aydintasbas.