The Democrats are stepping up the pressure on the Yingluck Shinawatra administration to speed up sales of paddy to pay farmers who took part in the rice-pledging scheme.
Party leader Abhisit Vejjajiva said the government should stop blaming others and pleading for sympathy over its failure to pay the farmers.
It should instead accelerate efforts to resell the pledged rice.
Caretaker Prime Minister Yingluck Shinawatra, in her capacity as chairwoman of the National Rice Policy Committee, should stop concealing acts of corruption in the rice scheme, Mr Abhisit said.
As a caretaker government, the Yingluck administration was not prohibited by any law or the constitution from reselling the pledged rice and paying the debts it owed to farmers, he said.
The only thing stopping the government from reselling the pledged rice now was that it was afraid to reveal the information to the public about the actual quality and the amount of pledged rice remaining in stock.
He also said a report about the recent cancellation of a deal in which China's state-owned Beidahuang Group agreed to buy 1.3 million tonnes of rice from the Thai government had also been enclosed in the Democrat Party's petition lodged with the National Anti-Corruption Commission (NACC) against the government.
Atthaporn Ponlaboot, a Democrat executive, said former Democrat MPs in central and western provinces would launch a campaign for the government to return rice pledged under its scheme to farmers if it could not pay for it.
In this case, the government would still have to compensate farmers for withholding their rice for long periods, he said.
Ms Yingluck was wrong to claim the rice scheme was not a failure and that its inability to pay the farmers was caused by the House dissolution and the Election Commission's refusal to approve the caretaker government's request to seek more loans to fund the scheme, Mr Atthaporn said.
In reality, he said, the scheme had failed due to corruption about a year before the House was dissolved.
Chawalit Wichayasuth, a list-MP candidate of the Pheu Thai Party, said he had not heard about any rice farmers asking to cancel rice-pledging agreements with the government so far.
Pheu Thai spokesman Prompong Nopparit said there was an attempt to topple the caretaker government and farmers were being instigated to join in the campaign.