But he reckoned without Muharrem Ince, the presidential candidate of the secularist Republican People's Party (CHP), whose feisty performance at campaign rallies has galvanized Turkey's long-demoralized and divided opposition.
Voting started at 0500GMT (8 am local time) and will continue through to 1400 GMT (5 pm local time). If a candidate wins just over 50 percent of the vote, he will win the presidency, but if not, there will be a runoff on July 8.
Six candidates are running for president: Recep Tayyip Erdogan for the People's Alliance (Cumhur Ittifaki), formed by Turkey's governing AK Party and the MHP, Muharrem Ince for CHP, Selahattin Demirtas for HDP, Meral Aksener for the Good (IYI) Party, Temel Karamollaoglu for the Felicity (Saadet) Party, and Dogu Perincek for the Patriotic (Vatan) Party.
Mr Erdogan called snap elections more than a year ahead of schedule, in a bid to usher in an executive presidency with sweeping powers.
If he does not achieve half the votes or more, the presidential election will move to a second bout.
More than 56 million people are eligible to vote across the country.
He was speaking at a rally in Istanbul on Saturday in the run-up to the early presidential and parliamentary elections scheduled to be held on Sunday.
The constitutional overhaul would mean that Erdogan could stay in power for another two terms until 2029.
More than 59 million Turkish voters have been voting since 6am this morning at over 180,000 polling stations that will close in about an hour's time.
Meanwhile, former physics teacher Ince built up a national following with lacerating attacks on Erdogan's rule, bringing up near-taboos like the AKP's past cooperation with the group of Fethullah Gulen blamed for the 2016 failed coup. According to the latest United Nations figures, some 160,000 people have been detained and almost as many more sacked in the crackdown.
She and others in the city said they voted for the pro-Kurdish Peoples' Democratic Party (HDP) so that it exceeds the 10 percent threshold of votes needed to enter parliament. If that happens it could cost Erdogan's AKP and its nationalist ally in the "People Alliance" dozens of seats - leading it to lose its parliamentary majority.
The president's critics, including the European Union which Turkey still nominally aspires to join, say Erdogan has used the crackdown to stifle dissent. "Backing the HDP means supporting democracy".