Parliament will spend at least two days this week deliberating charter amendments amid concerns they might run into a technical glitch.
Wirach Ratanasate, the chief government whip, confirmed on Monday that deliberation of the amendment bill in a joint sitting will begin on Tuesday if time permits.
The bill seeks to change the election method and restore the two-ballot system.
If the amendment bill's deliberation cannot be wrapped up by the end of tomorrow, the session could be extended to Thursday with a third and last reading vote held on Sept 10, according to Mr Wirach.
The amendment bill is sponsored by the coalition Democrat Party.
The legislation, which would raise the number of constituency MPs from 350 to 400 and restore the old selection formula for 100 list MPs, was the only one of 13 competing bills to sail through parliament.
However, some lawmakers were concerned parliament might agree on amendments beyond those agreed during the first reading.
The issue over whether the bill complies with the underlying principle might require interpretation by the Constitutional Court, which would threaten to sink it altogether.
Mr Wirach said such an interpretation may be sought after the bill passes its third reading.
The chief government whip added he did not believe the senators would shoot down the bill on the grounds of the amendment contents exceeding the principle. Certain details incorporated into the bill during scrutiny might be deemed "overboard" but they are negligible and would not harm the core value of the bill, he said. He did not elaborate.
Mr Wirach said he was confident the bill would be enacted in time for the next general election, which some critics speculate could be held early next year.
Senator Direkrit Jenklongtham, also a member of the House committee on charter amendments, said the lawmakers were split with some insisting that any changes to the charter must be restricted to amending Sections 83 and 91.
Section 83 governs the number of constituency MPs also any restoration of the old selection formula for 100 list MPs instead of the current 150.