The battle for support from 53 million registered voters comes against a backdrop of slow economic growth and heightened nationalist sentiment.
While more than 50 parties are contesting the polls, only three - the People's Party, Bhumjaithai, and Pheu Thai - have the nationwide organisation and popularity to gain a winning mandate.
A simple majority of the 500 elected lawmakers selects the next prime minister.
Local polls consistently project that no single party will gain a majority, necessitating the formation of a coalition government.
Although the progressive People's Party is seen as favoured to win a plurality, its reformist politics aren't shared by its leading rivals, which may freeze it out by joining forces to form a government.
The People's Party, led by Natthaphong Ruengpanyawut, is the successor to the Move Forward Party, which won the most House seats in 2023, but was blocked from forming a government by conservative lawmakers and then forced to dissolve.
Its platform continues to promise sweeping reforms of the military, police and judiciary, appealing to youth and urban voters. Legal constraints have led it to set aside demands for reform of a law that metes out harsh penalties for criticism of the monarchy, while putting new emphasis on economic issues.
Softening its politics risks weakening its core support, already at risk because the last election had positioned it squarely as the alternative to nine previous years of military-led government, a situation it can't fruitfully exploit this time.
At the same time, with the army's reputation burnished by the surge of patriotism that emerged during last year's border clashes with Cambodia, its positions critical of the military can be a political liability, said Napon Jatusripitak, director of the Center for Politics and Geopolitics at Thailand Future, a Bangkok-based think tank.
The Bhumjaithai Party, headed by incumbent Prime Minister Anutin Charnvirakul, is seen as the main defender and preferred choice of the royalist-military establishment.
Anutin has been prime minister only since September, after serving in the cabinet of former Prime Minister Paetongtarn Shinawatra, who was forced out of office for an ethics violation regarding mishandling relations with Cambodia. He dissolved parliament in December to call a new election when he was threatened with a no-confidence vote.
Subsequent border clashes with Cambodia allowed Anutin to recast himself as a wartime leader after his popularity initially slipped because of floods and financial scandals. His campaign focuses on national security and economic stimulus.
Bhumjaithai, seen as the likeliest party to form the next government, benefits from an electoral strategy employing old-style patronage politics and a machine skilled at grassroots organising in the vote-rich northeast.
The Pheu Thai Party, the latest political vehicle for billionaire former prime minister Thaksin Shinawatra, trades on populist policies innovated by its predecessor, the Thai Rak Thai party, which held power from 2001 until 2006, when it was ousted by a military coup.
Thaksin-backed parties staged repeated electoral comebacks, only to be ousted by conservative-leaning courts and state watchdog agencies. It softened its politics enough by the 2023 election to be returned to power after being judged by the previously hostile royalist-military establishment to an acceptable alternative to the more progressive Move Forward party.
The conservative court system rounded on it anyway - ousting two of its prime ministers over two years and ordering Thaksin imprisoned on old charges. The party now campaigns on economic revival and populist pledges like cash handouts, nominating Thaksin's nephew, Yodchanan Wongsawat, as its lead candidate for prime minister.
Sunday's voting includes a referendum asking voters whether Thailand should replace its 2017 military-drafted constitution.
The vote isn't on a proposed draft, but rather to decide whether to authorise parliament to begin a formal drafting process, which would require many further steps before coming to fruition.
Pro-democracy groups view a new charter as a critical step toward reducing the influence of unelected institutions such as the military and judiciary, while conservatives warn that it could cause instability.
