The debate is due to start on Tuesday.
Jirayu said the party would hold meetings at both the regional and national levels.
The meetings would discuss how many members the Constitution Drafting Assembly (CDA) should have – 99, 150 or 200, he said.
Democrat MP Thepthai Senapong, meanwhile, asked the government to allow members of Parliament to debate the issue freely. If a third reading could not be held in the current session, the session should not simply be extended, should wait until the next session, Thepthai said.
The current parliamentary session was originally scheduled to end on April 18, but Cabinet resolved last week to extend it indefinitely. The Democrats accused the government of trying to rush a charter amendment to help fugitive former premier Thaksin Shinawatra.
A joint House-Senate meeting to consider the second reading of changes to Article 291 of the Constitution is scheduled for Tuesday and Wednesday. Changes to the Article would allow the establishment of a CDA.
According to the law, the third reading cannot begin until 15 days after the second reading is passed.