* Court likely to rule against PM Yingluck, could affect her entire cabinet
* Yingluck's backers say the court is biased
* Supporters of PM and opposition prepare mass rallies
By Amy Sawitta Lefevre
BANGKOK, May 7 (Reuters) - Thailand's Constitutional Court rules on Wednesday whether to remove the prime minister from office for abuse of power, with an expected guilty verdict likely to bring her supporters into the street and plunge the country into a political vacuum.
Yingluck Shinawatra has faced six months of protests in the capital, Bangkok, aimed at toppling her government. The protesters have failed to achieve their aim in the street and are now looking to legal challenges to remove her.
"It is almost certain the court will rule against Yingluck. Her party will not accept that decision," said political analyst Kan Yuenyong at the Siam Intelligence Unit think tank.
"What is unclear is whether the whole cabinet will go with her. If it does, we will be left without a prime minister, a cabinet and a lower house. In short, Thailand will enter a void, a constitutional crisis."
The ruling is expected sometime after mid-day (0500 GMT).
The charges relate to Yingluck's transfer of National Security Council chief Thawil Pliensri in 2011, which opponents say was designed to benefit her Puea Thai Party and a family member.
Yingluck defended herself in court on Tuesday and denied benefiting from the transfer.
Some legal experts say the court may rule to remove ministers involved in that transfer decision or that her entire government will have to go if she is forced to step down.
Her party rejects that and says if Yingluck goes, an interim prime minister can be chosen from among her five deputies.
It remains unclear how a new premier would be appointed if Yingluck and her entire cabinet are forced to step down. There has been no lower house of parliament since December so it could fall to the Senate upper house.
"That prime minister would probably come from the opposition side which is another problem because the Puea Thai Party and its supporters will never accept that," said Kan.
The 150-seat Senate includes 77 elected members. The other 73 are appointed and are largely seen as opponents of Yingluck and her brother, former premier Thaksin Shinawatra, who is seen as the power behind her government.
The crisis broadly pits Bangkok's middle class and royalist establishment against the mainly poor, rural supporters of Yingluck and Thaksin, who was ousted by the military in 2006 and lives in exile abroad to avoid a jail sentence handed down in 2008 for abuse of power.
Yingluck's supporters accuse the Constitutional Court of bias in frequently ruling against the government. In 2008, the court forced two Thaksin-linked prime ministers from office.
Tensions between pro and anti-government supporters could come to a head if Yingluck is removed by the court.
Anti-government protesters are still massed in a central Bangkok park, albeit in diminished numbers. Yingluck's "red shirt" supporters plan a large rally near the capital this weekend, or earlier if she is forced to step down.
A group of red shirts plans a rally on Wednesday in the northeastern city of Khon Kaen in support of Yingluck.
The protesters disrupted a Feb. 2 election that Yingluck's party looked set to win. A court ruled the election void in March because voting was not possible in 28 southern constituencies where candidates were stopped from registering.
The government and the Election Commission have agreed to hold a new vote on July 20, but the cabinet has yet to draw up a draft decree for endorsement by Thailand's king.
Even if it goes ahead, it could again face disruption by the protesters, who are demanding political and electoral reforms before any vote in order to eliminate the influence of Thaksin.
Twenty-five people have been killed and scores wounded since the protests began in November. (Editing by Alan Raybould and Ron Popeski)