BANGKOK (Reuters) - Four high-profile cases involving key political players in Thailand are scheduled to go before the courts on Tuesday, in the latest legal wrangling that could see Southeast Asia's second-largest economy plunged into a new period of uncertainty.

The cases involve some of Thailand's most powerful politicians, including its current prime minister, and could deepen a decades-old rift between the conservative-royalist establishment and its opponents, such as the populist ruling Pheu Thai party and the opposition Move Forward party.

"The political parties and representatives that voters have chosen are being systematically and repeatedly stymied," Thitinan Pongsudhirak, a political scientist at Bangkok's Chulalongkorn University, told Reuters.

A single petition can bring down a sitting, elected government or oust a prime minister, he said, outlining the power of the country's courts.

"There's a judicial assertiveness that has been damaging to Thailand, subverting popular will and popular mandates."

Such tensions have previously triggered violent street protests, dissolutions of political parties, airport closures and military coups that have hamstrung the economy.

Thai stock markets have already been rattled by the spectre of a political crisis. The main stock index dropped to its lowest level since November 2020 on Monday, and has fallen 8.4% so far this year, making it Asia's worst-performing market.

The billionaire former premier Thaksin Shinawatra, 74, a powerful backer of the ruling government, could face pre-trial detention if he is denied bail on Tuesday after being formally indicted for insulting the monarchy in a 2015 media interview.

Separately, the Constitutional Court will conduct a hearing in a case lodged by a group of senators that could potentially see Prime Minister Srettha Thavisin dismissed from office for breaching the law in appointing a lawyer with a conviction record to his cabinet.

The same court will also hear a case seeking to disband the popular opposition Move Forward Party for their campaign to amend the country's royal insult law, following a complaint by the Election Commission.

The court is expected to announce the next hearing or verdict date for cases involving Srettha and Move Forward on Tuesday.

The Constitutional Court will also rule whether the ongoing selection process for a new upper house, which started earlier this month and is scheduled to conclude in early July, is lawful.

If the court cancels or delays the process, it would temporarily extend the term of military-appointed senators who have a played crucial role in the formation of the previous government.

(Reporting by Panu Wongcha-um, Editing by Devjyot Ghoshal and Sharon Singleton)

By Panu Wongcha-um