The numbers are staggering.

More than 100 million people are expected to vote, many for the first time. They’ll do so in booths across thousands of islands and three time zones, hammering nails into ballots to mark their choices. And within hours, if history is any guide, the world will know the outcome of the biggest race of the day: the one for Indonesia’s presidency.

Indonesia, the world’s third-largest democracy, will hold its general election on Wednesday. Election Day is a national holiday, and on average, about 75 percent of eligible voters have turned out. In addition to the president, voters are choosing members of Parliament and local representatives.

This election season has raised fears that Indonesia, which was an authoritarian state not long ago, is in danger of sliding back toward its dark past. The potential ramifications extend far beyond the country’s borders. As one of the world’s biggest exporters of coal, nickel and palm oil, Indonesia has a large role to play in the climate change crisis.

And in the contest between the United States and China for influence in Asia, Indonesia is seen by U.S. officials as a “swing state.” Under President Joko Widodo, ties with China have deepened significantly, but he has also maintained strong defense relations with Washington.

Here’s what you need to know.

What is at stake?

The election is widely seen as a referendum on the legacy of Mr. Joko, who is stepping down after two five-year terms.

Often referred to as Jokowi, he remains extremely popular because he has transformed Indonesia into one of Southeast Asia’s biggest economic success stories. He ushered in a universal health care system, built more than 1,000 miles of roads and highways, and oversaw respectable economic growth of about 5 percent a year.

His supporters say his job is unfinished and that there are pressing issues, such as inequality and poverty, that still need to be addressed. Critics say democratic norms have eroded under Mr. Joko’s time in power and that he is now maneuvering to extend his influence even after he leaves office.

Mr. Joko appears to be backing Prabowo Subianto, a onetime rival who has been accused of human rights abuses, to become his successor, alarming even some of his supporters. The outcome of the election could determine the future of democracy in Indonesia, which has the world’s biggest Muslim population.

Who’s running for president?

For the first time in 15 years, voters will get to pick from three presidential candidates: Mr. Prabowo, the current defense minister; Anies Baswedan, the former governor of Jakarta; and Ganjar Pranowo, who ran Central Java.

Mr. Prabowo has touted himself as the continuity candidate, saying this month that Mr. Joko’s policies had been “very, very beneficial for all of the people.” But he is a polarizing choice.

To many Indonesians, Mr. Prabowo is associated with the dictator Suharto, who ruled with an iron fist over the country from the mid-1960s to the late 1990s. Mr. Prabowo was married to one of Suharto’s daughters and served as a general in his military, which was notorious for human rights violations. In 1998, Mr. Prabowo was discharged from the army for ordering the kidnappings of student activists.

But Mr. Prabowo has shored up support thanks to an image makeover and the implicit backing of Mr. Joko. Surveys show Mr. Prabowo with a wide lead in the polls, but it is less clear whether he will win enough votes across a broad base of provinces to land the presidency without having to go through a runoff election in June.

A large number of swing voters, by one count around 13 percent of the electorate, make the result hard to predict.

Mr. Ganjar of Central Java — the candidate fielded by Mr. Joko’s political party, the Indonesian Democratic Party of Struggle — has also promised to continue most of Mr. Joko’s policies, albeit with tweaks. He has been described as “Jokowi lite.” But analysts say he has struggled to define his message, and polls show his support topping off at around 20 percent.

Mr. Anies, the former Jakarta governor, is highly regarded in the capital for improving public transportation and managing the coronavirus pandemic, but his previous ties to radical Islamist preachers have raised concerns.

In recent weeks, though, Mr. Anies has drawn support from Gen Z voters and educated urbanites on a message of change, with some surveys showing he is slightly ahead of Mr. Ganjar. He has argued that a plan pushed by Mr. Joko to move the capital to another island would not lead to equitable development, and he has warned about the return of nepotism.

Why is the youth vote so important this time?

People under 40 account for more than half of all eligible voters, making them the biggest bloc in this election. Surveys have found that younger voters are concerned about the economy, education, employment and corruption.

To reach this cohort, the presidential hopefuls have turned to social media. Mr. Prabowo, the former general, has tried to rebrand himself as a gemoy, or cute, grandfather. TikTok has been flooded with videos of him dancing at rallies. This strategy has endeared him to some voters.

Many young people are unaware of the problematic aspects of Mr. Prabowo’s past, such as his role in the kidnapping of activists, because the history of Suharto-era human rights violations is not taught in schools.

Independent groups that run websites like “Bijak Memilih,” or Choose Wisely, are working to help younger voters by providing news and information. One reason they are doing so is some young voters have expressed skepticism about the independence of the country’s media outlets.

Mr. Anies is also using social media to drum up support, turning to an unlikely bloc: Indonesian K-pop fans. Many such supporters say they were taken by Mr. Anies after he emerged from a debate and did a TikTok livestream with his supporters, where, like a K-pop star, he answered questions about his love life and his favorite books.

What sets this election apart from others?

It is one of the world’s most complex single-day elections. About 205 million people are registered to vote in this sprawling archipelago of about 17,000 islands, roughly 7,000 of which are inhabited.

Six million election officials have begun fanning out across the country to ensure that as many people as possible get a chance to vote. Logistics are a headache in some places — requiring officials to travel on horseback or take boats or helicopters and trek for hours to deliver ballots to voters.

“It is a massive, colossal task,” said Yulianto Sudrajat, a member of Indonesia’s General Election Commission who is in charge of logistics.

Voters will mark their ballots by hammering nails into them, which election officials say is a fairer method than using a pen, since some Indonesians are unfamiliar with writing instruments. As the votes are counted, election officials hold the ballots up so people can see light shining through the holes.

In 2019, the process took such a toll that 894 election workers died, prompting the government to urge volunteers this time to undergo health screenings.

Although the official vote count takes weeks to confirm, the results are generally known by the end of the day, based on so-called quick counts, a kind of exit poll. After polling stations close at 1 p.m. Jakarta time, independent pollsters will tally ballots from a sampling of voting stations nationwide.

In previous elections, the quick counts — released by 5 p.m. — have accurately reflected the real results.

Rin Hindryati and Hasya Nindita contributed reporting.