ISTANBUL — From soaring bridges to a giant mosque to plans for the world’s biggest airport, President Recep Tayyip Erdogan of Turkey has used gargantuan building projects as an engine of growth and a signature way of leaving an indelible stamp on his nation.
As he campaigns for re-election on Sunday, Mr. Erdogan has promised his most ambitious project yet: a canal that would bisect the country and create a Turkish-owned trade route, which he says would make Turkey a great power and leave a legacy for the history books.
“What makes Panama is the Panama Canal,” Mr. Erdogan told supporters at a rally in Istanbul last weekend. “Suez is the biggest source of revenue for Egypt. Let’s have a vote. God willing the Istanbul Canal will be another fresh breath for our city.”
The election is shaping up as an up-or-down vote on how Mr. Erdogan has transformed Turkey during 15 years in charge. He has amassed sultanlike powers, jailed political enemies and trimmed civil liberties, even as average annual economic growth of 5 percent has spawned and nurtured a middle class.
But the most obvious way Mr. Erdogan has left his mark stands before the eyes of any visitor: grandiose monuments and infrastructure investments in just about every town.
There are signs that the public is weary of Mr. Erdogan’s building mania. The canal is the latest dividing line between those who see Mr. Erdogan’s projects as visionary, and those who say the works are guided by an insatiable construction industry that has enriched his ruling circle, raising questions about his management of a faltering economy.
Mr. Erdogan called the election a year and a half ahead of schedule, hoping to beat the economic downturn nipping at his heels. A once-fractured opposition has united against him, making it increasingly uncertain whether Mr. Erdogan will meet the 50 percent threshold to win outright and avoid a runoff against his top challenger.
Mr. Erdogan counts his building feats at virtually every election rally and warns that his opponents plan to tear down everything his Justice and Development Party, or A.K.P., has built. The party “built 284,000 classrooms,” he declared recently in the town of Mugla, adding “Are you going to demolish them too?”
He lists his big canal project in first place on his campaign posters. Not one shovel has been put in the ground, but Mr. Erdogan has vowed to begin construction immediately if he is re-elected as president and assumes sweeping new powers.
All of his megaprojects have been about creating symbols of his strength as he aims for a place in the pantheon of great Turkish leaders, from the Ottoman sultans to the founder of the republic, Mustafa Kemal Ataturk.
But the 28-mile canal linking the Black Sea to the Sea of Marmara — estimated to cost $15 billion, though critics say the figure is closer to $65 billion, and displace some 800,000 people — has been dubbed his “crazy idea” since Mr. Erdogan first conceived it seven years ago.
“It means crazy, wow, in a good sense,” said Mehmet Akarca, head of Turkey’s general directorate for press and information and an adviser to the president. “It will make money, and ships will use it, and they will pay tolls to use it.”
That is the hope, at least, though many doubt whether it will ever happen — or whether it will work if it does.
Environmentalists warn that the canal would damage the ecosystem so much that Istanbul could become uninhabitable. Archaeologists caution that it would threaten a top-class Paleolithic site. Economists say the project is not financially viable.
“It’s like playing Moses,” said Serkan Taycan, an artist and opponent of the canal who has mapped the area that would be disturbed.
Ozgur Unluhisarcikli, the Ankara director for the German Marshall Fund of the United States, a research organization, credits Mr. Erdogan for building infrastructure that has helped Turkey’s rapid urbanization by linking cities to one another and to their suburbs. The construction sector has also provided millions of jobs to Turkey’s largely uneducated work force.
“One aspect of the big projects is that they are generating growth,” Mr. Unluhisarcikli said.
But Mr. Erdogan’s opponents say his economic model is dubious, even corrupt.
Abdullatif Sener, a former deputy prime minister, has alleged that Mr. Erdogan’s way of governing is all about the profit that the president and his close circle can gain in kickbacks.
Mr. Sener was a co-founder of the Justice and Development Party, as was Mr. Erdogan, but he resigned from the party in 2008 because of corruption, he says, and is now running for Parliament with the opposition Republican People’s Party.
“They don’t think about the concerns of the citizens,” Mr. Sener said of the government at a campaign rally in Aziziye, in central Turkey. “They think about ‘How can I make my friend, my family, my close circle, my buddy richer.’ With this mentality, this country could not escape disaster.”
Mr. Erdogan has accused his opponents of peddling lies. “We invested billions in Istanbul, and now they say we robbed the country?” he said last week at a rally in Istanbul.
Others criticize Mr. Erdogan for prioritizing construction over industry and trade, which would generate more income.
“We are not using the resources in the best way to earn money,” Durmus Yilmaz, a former chief of the Turkish central bank and a co-founder of a new opposition party, the Good Party, said in an interview at his home in Ankara.
“This is all financed through foreign borrowing,” he said. “Are these investments generating enough income so we can pay back the loans?”
Turkish industry has shrunk since 2002, when Mr. Erdogan first came to power — to 16 percent of gross domestic product from 22 percent — and the construction sector has grown in its place.
The decline has left new ports and tunnels underutilized and Turkey lacking enough exports to finance its ballooning foreign debt, Mr. Yilmaz said.
Then there are the extravagant projects — such as the presidential palace, four times the size of Versailles — that seem to be more about Mr. Erdogan’s legacy than profitability.
On Istanbul’s highest hill above the Bosporus, Mr. Erdogan is building the white marble Camlica mosque, appointing it with six minarets, the insignia of greatness.
Many wonder if he plans to build a mausoleum for himself beside his project, as did the sultans of old.
“Some are white elephants, that’s very clear,” said Refet Gurkaynak, a professor of economics at Bilkent University in Ankara.
The government has said that growth has been more than 7 percent in the last two quarters, but the economy is already stumbling, Mr. Gurkaynak said.
“We are in recession,” he said, and “we are going to have a painful recession.”
Housing construction has reached its limit, and two million apartments are unsold in the country. Construction work is grinding to a halt, and companies are offering real estate on soft loans or barter.
In such a climate, Mr. Gurkaynak said, “Canal Istanbul makes no sense whatsoever and will be impossible to finance.”
It does not help that the proposed canal route would run parallel to the Bosporus, where transit is free under the 1936 Montreux Convention.
Officials insist there will be enough traffic to make the canal profitable. But Mr. Gurkaynak and others argue that shippers would be unlikely to pay when there is a free passage a few miles away.
New oil and gas pipelines are already reducing tanker traffic through the straits, according to a report by Istanbul’s Chamber of Environmental Engineers.
The looming pitfalls are familiar. Two of Mr. Erdogan’s much vaunted bridges — the Osman Gazi Bridge over the Gulf of Izmit, and the Yavuz Sultan Selim Bridge, which spans the Bosporus — have little traffic partly because of the high tolls charged. The government is paying the shortfall.
Mr. Akarca, the presidential adviser, defended the projects’ financing system, which has been mostly a variety of public-private partnerships.
“Turkey actually is not borrowing any money,” he said. “These are the Turkish firms that are borrowing money; it is individual debt.”
But the government has guaranteed the loans and the revenue, so some economists have said that the financing model is enriching private firms while saddling the country with debt.
Other resistance to Mr. Erdogan’s building spree is centered on the environment and conservation.
To the horror of archaeologists, Mr. Erdogan started the $4 billion Marmaray railway project beneath Istanbul’s historic peninsula, and built a highway along the Byzantine city walls of a World Heritage site, over Unesco protests.
Residents have pushed back against projects that have favored construction magnates, most prominently at the central Taksim Square, where huge protests exploded in May 2013 to save the park from commercial developers.
The protests, which left eight people dead and hundreds more injured, drew many citizens to oppose Mr. Erdogan’s headlong drive to modernize Turkey’s cities.
Yet instead of listening to them, Mr. Erdogan doubled down on his projects. Taksim became a symbol of his determination to impose his will.
“He needed opponents and victories and symbols,” said Mucella Yapici, an activist and a member of Istanbul’s Chamber of Architects. “Taksim is the most important.”
Last month, Istanbul’s cultural center, an emblem of Ataturk’s openness to the West, was pulled down at the site, and a vast domed mosque raised opposite it, dwarfing Ataturk’s statue and robbing the square of its republican nature.
“This is an attempt to erase the collective memory of the space,” Ms. Yapici said.
Much more stands to be erased in the canal project — entire towns and villages, as well as the ecology of Istanbul’s main water source.
The greatest concern is the potentially huge inflow of nutrient-rich water from the Black Sea, which scientists say would encourage the growth of algae and kill life in Sea of Marmara.
The Bosporus is so deep that it allows a countercurrent. The canal would have no such balancing effect.
One scientist has warned that Istanbul will come to stink of bad eggs from hydrogen sulfide. Other environmentalists warn that the vital wetlands used by migratory birds will be destroyed.
The government held a single meeting in March with landowners to introduce an environmental impact assessment. It claimed the canal would have negligible effect.
Istanbul’s Chamber of Environmental Engineers produced its own assessment, warning that the canal project — which includes plans for a new city for as many as three million people — would cause irreversible harm.
“In the long run we will lose the Sea of Marmara and do damage to the Black Sea,” said Sedat Durel, an environmental engineer who worked on the report.
Mr. Durel estimated that as many as 800,000 people would be displaced.
They include several thousand Crimean Tatars, refugees from the Crimean War who settled in the Sazlidere Valley west of Istanbul 150 years ago, after being granted the land by the sultan of the Ottoman Empire.
Their descendants, mostly farmers and factory workers, are anticipating with some dread any official orders to be uprooted again.
“Of course the canal is important,” said Oktay Teke, the mayor of Sazlibosna, a village amid meadows beside the river. “But what about all these villages? What will happen?”