Thais voted for half of the country's 150-seat Senate in a key test for Yingluck Shinawatra's troubled government, a day before the prime minister is due to defend herself against negligence charges over a disastrous rice subsidy scheme.
Anti-government protesters are in their fifth month of a campaign to force Yingluck out and set in motion political and electoral reforms before a new general election takes place.
Yingluck's opponents want impeachment charges brought against the country's leader over the government's financially ruinous rice scheme.
A Senate dominated by anti-government politicians could hasten her exit. Thailand's 150-seat Senate is made up of 77 elected senators.
The other 73 seats are appointed and are seen as allied to the anti-Thaksin establishment.
Anti-government forces want to ensure a conservative, pro-establishment, majority to influence any decision to remove Yingluck which would require the votes of three-fifths of the senators.
Yingluck is due to appear before the National Anti-Corruption Commission (NACC) on Monday to defend herself against charges of dereliction of duty for her role in overseeing the botched rice scheme.
While party affiliation is prohibited in the non-partisan Senate, the majority of the 77 elected seats will be decided on the basis of endorsements from powerful, party-affiliated, local institutions, particularly in rural areas, meaning that the result could deliver a pro-Yingluck majority.
''The Senate vote is likely to deliver a result similar to the nullified February 2 election for the lower house, meaning it will be pro-government,'' said political analyst Kan Yuenyong at Siam Intelligence Unit.
''However most, around 90 per cent, of appointed senators are anti-government so if the Senate is asked to remove Yingluck they're very close to the number of voices needed to do that.
''Appointed senators are chosen by a committee that includes the heads of the National-Anti Corruption Commission, Constitutional Court, Election Commission, State Audit Commission and a representative of the Supreme Court.
Government supporters accuse the courts of bias and say many judges are aligned with the conservative establishment.
Thailand has been locked in a seemingly intractable political stalemate since Thaksin was ousted in a 2006 coup. The conflict broadly pits the Bangkok-based middle class and royalist establishment against the mostly poorer, rural supporters of the Shinawatras.
Protesters disrupted a February 2 general election which was nullified by a court on March 21, leaving Thailand in political limbo and Yingluck at the head of a caretaker government with limited powers.
Election officials have said it will take at least three months to organise a new vote but that election going smoothly looks unlikely.
Protesters have vowed to disrupt any general election held before their political changes are enacted.
Tens of thousands of anti-government protesters marched through the streets of Bangkok on Saturday to drum up support to oust Yingluck and rid the country of the influence of her brother, former premier Thaksin Shinawatra.
''If the opposition keeps boycotting elections we'll never get anywhere and Thailand will be unable to solve its problems,'' Jarupong Ruangsuwan, leader of the ruling Puea Thai Party, told reporters after casting his vote for the upper house on Sunday.