"There's no good reason to move the vote from Monday. This will make society feel that the Senate is under the government's influence," said Sen. Rosana Tositrakul, one of Thaksin's harshest critics.
While it would appear to be in the short-term interests of Thaksin's opponents to try to vote down the bill as quickly as possible, that could take the wind out of what has been a surprisingly strong and broad-based revival of the anti-Thaksin movement, which helped spark the 2006 military coup.
Thaksin's foes also helped force two pro-Thaksin prime ministers out of office in 2008. During that year, the "Yellow Shirts" occupied the prime minister's office complex for three months and Bangkok's two airports for a week.
Thaksin's sister and current Prime Minister Yingluck Shinawatra made several attempts in televised addresses this week to assure protesters that the ruling Pheu Thai party will not push to pass the legislation again if the Senate strikes it down. The more-powerful lower house legally can pass legislation without Senate approval after a 180-day wait.
Sixty-nine of the Senate's 149 members attended Parliament on Friday. They had announced a tentative agreement that all would vote against the bill.
The bill's original draft, approved in principle by the lower house in August, did not extend the amnesty to the leaders of pro- and anti-Thaksin groups, but a committee in mid-October changed the bill to include them, leading to criticism that it was intended all along to apply to Thaksin. The altered bill was passed by the lower house last Friday.
Thaksin was ousted by the military over allegations of corruption and disrespect for King Bhumibol Adulyadej.
The bill has also drawn opposition from some Thaksin supporters who oppose immunity for Democrat Party leader and former Prime Minister Abhisit Vejjajiva and his deputy for their alleged roles in the deaths of scores of pro-Thaksin "Red Shirt" protesters in a bloody 2010 crackdown.