The poll is the first in the Southeast Asian country since a youth-led uprising in 2020 that broke long-held taboos by calling for curbs on the powers of King Maha Vajiralongkorn, as well as an end to a near-decade of military-backed rule.

The vote on Sunday is expected to deliver a strong mandate for change, with public opinion surveys consistently predicting a majority for the main opposition Pheu Thai Party and the youth-led Phak Kao Klai (Move Forward Party, MFP).

But fears have remained that the royalist-military establishment may seek to cling on to power. In the past 20 years alone, the military has staged two coups while the courts have brought down three prime ministers and dissolved several opposition parties.

“People are worried and they are scared,” said Hathairat Phaholtap, the managing editor of the Isaan Record newspaper. “They have waited for this vote for so long, and it means a lot to them. There’s a lot of tension, but also excitement and hope.”

Here’s what you need to know about Sunday’s election.

Who are the main contenders?

Leading in the polls is Pheu Thai (For Thais), the opposition party aligned with self-exiled billionaire Thaksin Shinawatra, whose removal in a coup in 2006 set off Thailand’s political turmoil. Despite Thaksin’s fall, parties linked to the telecomms tycoon have won every election since, including twice in landslides.

The strong election showings came on the back of pro-poor policies such as universal healthcare and debt relief for farmers.

This year, Pheu Thai is again pledging to expand welfare programmes and stimulate Thailand’s pandemic-stricken economy, including by offering 10,000 baht ($300) as handouts for those aged 16 and above.

The party is currently headed by Thaksin’s 36-year-old daughter, Paetongtarn Shinawatra.

At Pheu Thai’s final rally on Friday, Paetongtarn urged thousands of red-clad supporters to help the party win by a landslide to end the military-backed “dictatorship” and “better the lives of the people”.

Close behind Pheu Thai in the polls is MFP, led by 42-year-old businessman Pita Limjaroenrat.

The progressive party has put democratic reforms at the centre of its agenda, including pledges to scrap Thailand’s military-drafted constitution, abolish military conscription and revise the country’s strict lese majeste laws, which punish insults to the king with up to 15 years in jail.

The charismatic Pita – who has drawn large youthful crowds at his campaign events – has seen a surge in support in recent weeks, with the latest polls showing that the public favour him for the position of prime minister over Paetongtarn.

“Our time has come,” Pita told thousands of orange-clad fans at MFP’s final rally in the Thai capital, Bangkok. “To end Thailand’s political crises, we have to end the cycles of coups – for good.”

Facing off against the two reform parties is Prime Minister Prayuth Chan-ocha’s Ruam Thai Sang Chart (United Thai Nation Party, UTN).

The nationalist conservative party, which champions values such as peace, harmony and respect for the monarchy, is trailing in the polls at a distant third.

Prayuth – who first seized power in a coup in 2014 – wrapped up his campaign on Friday by cautioning supporters against “harmful” and “revolutionary change”. He also appealed to the mostly older crowds to protect the “values of Thais”.

How do the elections work?

Some 52 million people of Thailand’s 71 million people are eligible to vote in Sunday’s vote.

Up for grabs are the 500 seats in the House of Representatives. These include 400 seats that are directly elected and 100 seats that are allocated on a proportional representation basis.

Voters will be given two ballots, one for their local constituency and the other for their preferred party on a national level.

Polls will open at 8am local time (01:00 GMT) and close at 5pm (10:00 GMT).

When will the results be known?

The first unofficial results will start trickling in within hours of polls closing.

The Elections Commission said on Thursday results from the 95,000 polling stations nationwide will be compiled, verified and published on its website from 7pm (12:00 GMT) onwards on voting day.

The commission expects unofficial results to be known by 11pm (16:00 GMT) that same evening.

It has two months to formally ratify the election outcome.

How is a prime minister chosen?

Parties must win 25 seats in the lower house to nominate a prime minister.

Polls suggest Pheu Thai is on track to take about 220-240 seats in the 500-member chamber, while MFP will probably win between 70 and 100 seats.

The two parties have indicated a willingness to work together, but even with their combined total, they may struggle to form a government.

This is because the military-drafted constitution allows an unelected 250-member Senate to participate in the vote to appoint the prime minister.

So candidates must win the support of more than half of the combined houses, or 376 votes, to take the top job.

Pheu Thai and MFP look set to fall short of that number.

Therefore, analysts have said Prayuth’s return as prime minister, despite his party’s dismal standing in the polls, cannot be ruled out. After all, it was the same Senate that unanimously helped elect Prayuth to the post in 2019, as the head of a 19-party coalition.

For that reason, many will also be watching the smaller parties.

These include the Palang Pracharat Party (People’s State Power Party), led by Prayuth’s deputy Prawit Wongsuwan, and Bhumjaithai Party (Thai Pride Party), which has strong regional backing in northeastern Thailand.

What are the possible outcomes?

Analysts saw three main possible scenarios; Prayuth’s return with the support of the Senate, a coalition between Pheu Thai and MFP, or a partnership between Pheu Thai and the smaller Palang Pracharat Party.

The first scenario would result in a minority government.

“This would mean a rickety government, legislative gridlock, and government collapse during key votes,” said Thitinan Pongsudhirak, professor of international relations at the Chulalongkorn University in Bangkok. “Yet the Prayut-fronted regime may be desperate to keep Move Forward and Pheu Thai out of power, preferring to cross one bridge at a time.”

The second possibility may not even work out.

The appointed senators are likely to block a Pheu Thai-MFP government due to their opposition to the smaller party’s radical reform agenda.

That leaves a potential coalition between Pheu Thai and Palang Pracharat.

“The third plausible case is the most practical,” said Thitinan. Palang Prachat’s leader, Prawit, is a former general and a deal between the two parties “would break up the Senate vote and possibly be palatable to the palace”.

Amid all the uncertainty, what is clear is that the government formation process looks set to be a drawn-out process.

Thai voters may have to wait weeks, possibly several months to find out what their next government will look like.