A joint sitting of the parliament on Wednesday rejected a charter amendment bill which would have ended senators' involvement in the election of the prime minister.
The bill was presented by Somchai Srisutthiyakorn of the opposition Seri Ruam Thai Party.
It needed a majority of active elected members of the parliament, at least 364, and at least one-third of senators, or 84.
The bill received votes in favour from 333 representatives and 23 senators, while 102 representatives and 151 senators voted against it. Eight representatives and 45 senators abstained.
The 2017 constitution empowered senators to co-elect the prime minister along with House representatives during the first five years after the installation of the first parliament under the constitution.
Pita Limjaroenrat, leader of the opposition Move Forward Party, said senators' involvement in the election of the prime minister was not democratic and his party would campaign for a referendum asking people if the country should have a new constitution.
He also asked senators to vote for the prime ministerial candidate who reflected the outcome of the general election.
Opponents to the senators' involvement in the process said 250 senators were hand-picked by the dissolved National Council for Peace and Order headed by Gen Prayut Chan-o-cha, who is now the prime minister.