ISTANBUL—Turkish voters headed to the polls Sunday to decide whether they want more of Recep Tayyip Erdogan as president after 15 years of increasingly authoritarian leadership.
As he runs Turkey under a state of emergency crackdown imposed after a failed coup in 2016, Mr. Erdogan has concentrated enormous powers in his hands. Some media covered the presidential and parliamentary election campaigns as if he was the sole candidate.
But pollsters say the 64-year-old politician, who boasts he has never lost a ballot, is facing serious challenges.
A tumbling Turkish lira, which has lost a quarter of its value against the dollar since the start of the year, has left voters questioning Mr. Erdogan’s claim he knows the road to economic prosperity. And despite little access to media, which is kept under the government’s thumb, a credible rival has emerged.
Recent surveys showed Mr. Erdogan winning the presidency by a razor-thin margin in the first round or with a wider lead in a July 8 runoff against Muharrem Ince, candidate for the secular Republican People’s Party and likely second finisher. But they also indicated his ruling Justice and Development Party and its nationalist ally would fail to secure a majority in parliament.
“The race is more open than it seems,” said Hakan Bayrakci, head of Turkish polling agency Sonar.
If Mr. Erdogan wins but his alliance fails to garner enough votes to govern alone, Turkey could be plunged into a period of uncertainty as the country helps Europe control a migrant influx and repositions itself as a power broker in the Syrian war.
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Raising the stakes, a re-elected Mr. Erdogan would gain vastly expanded executive powers to shape legislation and the judiciary thanks to constitutional changes voters narrowly approved last year. But if opposition forces were in control of parliament they could use the assembly to challenge his authority.
Mr. Erdogan has suggested he would try to broaden his group by adding forces from the opposition if his own parliamentary alliance had no majority, a move he has managed before.
“There is nothing forbidding coalitions among alliances,” he told Turkish radio on Wednesday.
Mr. Erdogan cast his vote in Istanbul, where supporters greeted him with chants and applause.
“Stand upright, the nation is by your side,” said the crowd gathered near the school turned polling station.
Among the supporters, Beytullah Altınogulları, a 30-year old construction worker, said Mr. Erdogan was “the captain” Turkey needed.
“The economy may not be going well,” he said. “But Mr. Erdogan is the only man who would do everything better for Turkey.”
Mr. Erdogan called the vote in late April despite the fact that elections weren’t due until the end of 2019, catching the country and rival contenders off guard. For weeks, he campaigned with little apparent competition, unveiling grandiose construction plans, such as a 25-mile canal parallel to the Bosporus and announcing a $6 billion package comprising a tax amnesty and special allowances for retirees.
In a surprise development, however, the CHP candidate Muharrem Ince has galvanized large crowds of supporters across the country, and dueled with Mr. Erdogan through rally speeches.
Days after Mr. Erdogan mocked him as “the poor guy” CHP has lined up, Mr. Ince said he was indeed the son of a poor truck driver, adding: “It is better to be poor than to earn dirty money.”
When Mr. Erdogan proposed building libraries with free tea and cakes, Mr. Ince said “If you want to eat free cakes, vote for Mr. Erdogan, if you want factory jobs, vote for me.”
Mr. Erdogan, who remains Turkey’s most popular politician thanks to strong support among modest households and small-business owners, has long benefited from a fragmented opposition.
But in another surprise development, forces that are far apart on the political spectrum—CHP, the newly created nationalist Good Party and the pro-Islam Felicity Party—have joined together to oppose Mr. Erdogan.
The main pro-Kurdish movement, the Peoples’ Democratic Party, which has nearly 9% of the seats in the current assembly, hasn’t ruled out working with the alliance.
The opposition coalition and independent observers maintain that a referendum held last year that gave the presidency more powers was marked by a high number of irregularities—while opening ballot boxes, authorities declared that unstamped ballots would be regarded as valid. For this vote, the opposition said it has focused on assembling teams of observers and a set of cellphone apps to ensure it could monitor balloting and vote count.
On Sunday, the election board said it was investigating reports of security incidents at polling stations in regions bordering Syria. Meantime, state-run news agency Anadolu said several foreigners claiming to be international monitors had been detained.
Mr. Ince urged voters to keep going to the polls despite the reports, and called on them to help watch counting procedures.
“I will protect your votes at the cost of my life,” he said in a Twitter message.
Asked about alleged incidents, Mr. Erdogan said “there are no serious problems.”
One app designed by the opposition sends alarm signals in case observers aren’t present at the voting stations assigned to them, alerting party officials that an incident may have occurred.
“We will make our best effort to ensure that votes will come out of ballot boxes the same way they came in,” said Yusuf Erciyas, a senior member of CHP responsible for the vote-monitoring plan.
Write to David Gauthier-Villars at David.Gauthier-Villars@wsj.com and Yeliz Candemir at yeliz.candemir@wsj.com