The Senate is likely to reject the controversial blanket amnesty bill when it reaches the Upper House next week.
Informed sources said many senators have already agreed that they will not vote for the bill in the first reading.
"We will reject it as we cannot accept the principle of granting a blanket amnesty to all wrongdoings dating back to 2004.
As the principle is not acceptable, there is no point amending it in the second reading," said a senator who asked not to be named.
He estimated 70-80 senators would vote against the bill, which is expected to be deliberated by the Upper House next Monday.
He admitted the Pheu Thai government has been lobbying senators to support the bill. Many senators have been in favour of the government in the past including lending support to amending the charter, which is now pending a Constitution Court ruling over whether the draft law is constitutional.
However, for the amnesty bill, sources said it is "too much" to grant such a blanket amnesty.
The support of at least 76 out of 150 senators is needed to pass or reject the blanket amnesty bill.
Somchai Sawaengkarn, a member of the so-called Group of 40 Senators, said many senators are opposed to the revised version of the bill.
"We believe the Senate can defuse tension if we pay heed to the people's call," he said.
If the bill fails to clear the Senate in its first reading, the House of Representatives will have to wait at least 180 days before it can reaffirm its decision to pass the bill into law. Mr Somchai said he doubted MPs would push for the bill if the Senate rejects it.
However, if the bill passes its first reading, he said the Group of 40 Senators will try to change its content in the scrutiny stage before the second and third readings. "If it sails through the Senate, the next battle is in the Constitution Court," he said.
Mr Somchai's comments coincide with a statement issued by a group of 70 senators calling themselves "Senators against the Amnesty Bill".
The group is said to be a coalition of Group of 40 Senators and senators in central and southern provinces.
The group has pledged to vote against the bill which it claims will set bad precedents, and cause injustice and divisiveness in society.
"[The group] will vote against the bill and work with the public to campaign against it," the statement said.
According to the group, the government is not authorised to grant an amnesty to people who are charged with criminal offences. Those offences are committed against the rights of others.
The bill also "takes the opportunity" to grant an amnesty to people who are charged with corruption. Moreover, it will nullify a number of court rulings, which interferes with judicial powers.
As an agency which scrutinises bills, the Senate should not allow the amnesty bill to pass, said the statement.
Concerns were raised on Monday during a Senate meeting chaired by Surachai Liangboonlertchai about how the Senate should proceed.
Uttaradit senator Naruemon Siriwat asked the meeting to clarify if the bill was set for three straight readings as speculated.
Senator from Tak province Charin Harnsuebsai said he intended to vote for Mr Worachai's bill yet changed his mind after the draft was altered significantly.