Future Forward Party leader Thanathorn Juangroongruangkit has faced a barrage of criticism for his controversial remark on scrapping the constitution drafted by the regime.
"The 2017 constitution can't be amended so it will have to be torn," he said on Sunday after members voted him to lead the party during a meeting at Thammasat University's Rangsit campus.
He also planned an amnesty for all political prisoners charged by the National Council for Peace and Order (NCPO).
The businessman-turns-politician said the charter was a product of dictatorship under the NCPO.
His comment was met with caution from key figures at Government House and other parties on Monday.
Prime Minister Prayut Chan-o-cha said it was inappropriate to criticise the charter and blamed it for political woes. The NCPO will keep an eye on political parties if they violated laws, he added.
His deputy, Wissanu Krea-ngam, warned the young politician to take responsibility for what he had said although it was not a party platform. "We have to 'underline' that word," he said, when asked about Mr Thannathorn's idea to put an end to the constitution, "because it's serious and could pose problems."
"Everybody has to take responsibility for what he says," he added.
Wirat Kanlayasiri, the lead legal expert of the Democrat Party, advised Mr Thanathorn to study the constitution on voting and the amendment procedure first, instead of offering an "empty promise to lure voters".
Democrat deputy spokeswoman Mallika Boonmeetrakul warned that the amnesty and charter-scrapping ideas could bring back the political rifts experienced earlier.