A joint sitting of parliament rejected a charter amendment bill providing for "a people's constitution" at its first reading on Wednesday.
The bill received only 206 votes from MPs and senators, with 473 votes against it. Three MPs and three senators abstained.
The bill required 362 votes from the joint chambers, including one-third of the Senate, or 83 senators, to pass its first reading.
It received only three votes of support from senators. They were Naowarat Pongpaiboon, Pisan Manawapat and Monthian Boontan.
Democrat Party MP, Panit Wikitset, was among the three MPs who abstained.
The bill had the support of more than 135,000 eligible voters and called for key changes in the 2017 constitution. It was sponsored by the Internet Law Reform Dialogue, Re-Solution group, the Progressive Movement and the Move Forward Party.
One of the most controversial proposals was to abolish the Senate, leaving only the House of Representatives in parliament.
Supporters of the bill said the Senate was the product of the coup-makers' National Council for Peace and Order, since dissolved.
Senator Kamnoon Sidhisamarn, spokesman for the Senate whip, argued it would allow MPs to monopolise lawmaking. Senators also denied they were proxies of the coup regime.
Piyabutr Saengkanokkul, secretary-general of the Progressive Movement, and Parit "Penguin" Chiwarak, a key protest leader who spearheaded the movement behind the people's draft, reacted differently to yesterday's vote result.
While Mr Piyabutr kept urging supporters of the rejected charter amendment bill to fight on to achieve their goal of amending the constitution, Mr Parit was more crestfallen.
"It's been almost three years since the 2019 election and only one charter amendment bill has been passed in all three rounds of parliamentary deliberation of such drafts," said Mr Parit, referring to the charter amendment bill on the election system.
"For those who support this draft amendment, please don't lose hope [just because the draft was rejected]. As long as we continue to fight, we still have a chance to win," said Mr Piyabutr.