National Anti-Corruption Commission spokesman Vicha Mahakun said today that the agency found the lawmakers had misused their authority by seeking to amend the now-defunct constitution to make the Senate fully, rather than partly, elected.
The Constitutional Court ruled in 2013 that the attempt to change the composition of the 150-member Senate was unlawful.
Vicha said the agency will submit its evidence to the interim National Legislative Assembly, appointed after a military coup last year.
A vote for impeachment could ban the former lawmakers from political office for five years.
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Although the former senators had voted the same way as the former lower house members, they are technically non-partisan with no political party affiliations.
Members of the lower house were active members of a party loyal to Thaksin. The current military-led government is seeking to crush Thaksin's political movement.
The moves by the anti-corruption body, along with various court rulings, are widely seen as attempts to cripple the political machine of Thaksin, who was overthrown in a coup in 2006, and to prevent his allies from returning to power.
She was impeached earlier this year in connection with a money-losing rice subsidy scheme, barring her from office for five years. The attorney general is pressing criminal charges against her over the same matter.
Thaksin's supporters believe the traditional establishment , led by royalists and the military, is jealous of his electoral popularity and fear losing power and influence.
A constitution currently being drafted is expected to strengthen the permanent bureaucracy, which is associated with the royalist establishment, at the expense of political parties.