Thais voted yesterday for half of the country's 150-seat Senate in a key test for Yingluck Shinawatra's troubled government.
The prime minister is due to defend herself today against negligence charges over a disastrous rice-subsidy scheme.
Anti-government protesters are in their fifth month of a campaign to force out Yingluck and set in motion political and electoral reforms before a new general election. They want impeachment charges to be brought against her 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-Yingluck 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 before the National Anti-Corruption Commission (NACC) today 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.
Appointed senators are chosen by a committee that includes the heads of the NACC, Constitutional Court, Election Commission, State Audit Commission and a representative of the Supreme Court.
Government supporters accuse the courts of bias and claim that many judges are aligned with the conservative establishment.