AMNA NAWAZ: This Sunday, more than 60 million Turks will cast their ballot to decide the country's next president.

Turkey is a NATO member, part of the G20, and for two decades has been led by Recep Tayyip Erdogan.

The opposition calls him authoritarian.

He calls them terrorists.

And, as Nick Schifrin reports, Erdogan faces his toughest battle yet in an election that could have major implications for Turkey and its allies.

NICK SCHIFRIN: In the city that propelled him to power three decades ago, the man dubbed Turkey's new sultan unleashed new insults.

Turkish president Recep Tayyip Erdogan last weekend branded his opponent pro-LGBT, a supporter of terrorism, and a drunkard.

RECEP TAYYIP ERDOGAN, Turkish President (through translator): Mr. Kemal, drink as much as you want.

You can drink a full keg, but even that will not make you better.

My nation will not give the floor to an alcoholic.

NICK SCHIFRIN: Mr. Kemal is opposition candidate Kemal Kilicdaroglu, who focuses on kitchen table issues from his kitchen table.

KEMAL KILICDAROGLU, Turkish Presidential Candidate (through translator): This is the real agenda of the people, as they all know that, when I come to power, there will be democracy, and your purchasing power will increase.

NICK SCHIFRIN: Kilicdaroglu has pressed that message at his own massive rallies.

And a unified opposition of a half-a-dozen parties poses the greatest electoral threat that Erdogan has ever faced.

GONUL TOL, Founding Director, Middle East Institute Center for Turkish Studies: I know it's a cliche for the politicians to say that the upcoming elections are existential, but, in Turkey's case, it's actually true.

NICK SCHIFRIN: Gonul Tol is the founding director of the Middle East Institute's Turkey Program and the author of "Erdogan's War."

GONUL TOL: If he wins another term, I think Turkey will degenerate further into an authoritarian regime.

If the opposition wins, however, I think Turkish democracy will have a shot.

NICK SCHIFRIN: Voters' number one issue has been sticker shock.

Today, inflation is more than 40 percent, down from 85 percent last November.

MELIKE SAKA, Deli Owner (through translator): They have said inflation is 85 percent.

But I don't think it is 85 percent.

It feels around 250 percent to 300 percent.

NICK SCHIFRIN: Thirty-one-year old Melike Saka feels the cost of living crisis as consumer and seller.

She works at a deli that's a family affair.

That's her father.

They have never witnessed consumers buying less or profits dropping faster.

MELIKE SAKA (through translator): The quantities that we sold have decreased.

I can say our sales have decreased by half.

NICK SCHIFRIN: Turkey's economic pain runs deep, but analysts say Erdogan's unorthodox prescription of slashing interest rates made the country sicker.

GONUL TOL: The country's faltering economy and the cost of living crisis has been high on the voters' agenda.

And these economic troubles are largely of President Erdogan's own making.

And the second problem for voters is this growing repression.

NICK SCHIFRIN: In 2013, police and the government cracked down on critics after massive anti-government protests.

In 2016, the government accused the military of a failed coup, and Erdogan launched a society-wide purge, 50,000 arrested; 150,000 lost their jobs or were suspended, including teachers branded terrorists, who clashed with police.

And critical journalists like "Cumhuriyet Books" magazine editor Turhan Gunay were detained, as he told me back in 2017.

What happens to people in Turkey right now if they oppose the government?

TURHAN GUNAY, Editor, "Cumhuriyet Books" (through translator): I can only answer this question through my own experience, and that is, you are thrown into jail.

The government has no tolerance for the slightest criticism.

NICK SCHIFRIN: And the 2018 election created a new presidential system that increased Erdogan's powers.

RECEP TAYYIP ERDOGAN (through translator): From now on, it's time for the president and the Parliament to do more work.

NICK SCHIFRIN: But that portrayal of himself crumbled in February's earthquake.

Countless buildings collapsed.

At least 50,000 people died.

Survivors criticized the government for a slow response and for favoring builders who cut corners.

GONUL TOL: He promised that, if the country switched to a presidential system that would grant him unprecedented powers, that he would be able to solve the country's pressing problems in a more efficient way.

And on the day of the earthquake, we saw that that was not the case.

NICK SCHIFRIN: It was another megaquake in that helped propel him to 1999 power.

He became prime minister in 2003 thanks to anger against the secular political elite.

He was the alternative, a religious, working-class outsider who presented himself a progressive ally of the West, and helped lead the country to economic growth.

But, today, Erdogan's charisma and populism has divided society.

GONUL TOL: He has become a very polarizing figure.

And that's why I think he's facing such a huge challenge in the upcoming vote, because he always relied on others, relied on alliances to secure a majority, and for the first time in two decades, he's struggling to find new allies.

NICK SCHIFRIN: He's leaned on strongmen, including Russian President Vladimir Putin, for economic, political, and military support.

An Erdogan loss could weaken that partnership.

And the opposition fears an Erdogan loss could also lead to violence.

On Sunday, a leading opposition figure's bus was pelted with stones, and Erdogan has threatened to take to the streets.

Today, Kilicdaroglu's aides said he campaigned with a bulletproof vest and armed guards.

He and his wife have made a heart their symbol.

And he hopes to put an end to what he calls 20 years of one-man rule.

For the "PBS NewsHour," I'm Nick Schifrin.