The Bhumjaithai Party has admitted defeat in Sunday's by-election in one constituency of the southern province of Nakhon Si Thammarat, in which the Klatham Party secured victory.
However, another defeated candidate has vowed to seek a probe into alleged vote-buying.
Klatham candidate Kongkiat Ketsobat unofficially won the by-election in Constituency 8 with 38,680 votes.
The polls were held to find a replacement for Bhumjaithai MP Mukdawan Luengsinil, who was sacked by the Criminal Court for vote-buying. She is now serving a 10-year political ban.
Labour Minister Phiphat Ratchakitprakarn, a key figure in Bhumjaithai, attributed the party's defeat to Mr Kongkiat's tremendous popularity in Chawang district, a part of Constituency 8 and also the candidate's hometown.
Mr Phiphat noted that Bhumjaithai did better this time in the constituency than during the general election. It received over 28,000 votes, compared to 23,000 earlier.
Party leader Anutin Charnvirakul described Sunday's elections as being "clean and fair" and downplayed speculation that the South would serve as a new political battleground between his party and Klatham.
"This is not a do-or-die issue because the people will decide. I have sent a text message to congratulate the leader of the Klatham Party," said Mr Anutin, who also serves as a deputy prime minister and interior minister.
Agriculture Minister Narumon Pinyosinwat, leader of the Klatham Party, said the party is working hard to increase the number of members and constituency representatives to better serve Thais nationwide, not only in the South.
She dismissed speculation that the party's victory would increase the negotiating power for more cabinet seats, saying that was not its goal as a coalition partner.
Meanwhile, Chinnaworn Boonyakiat, a defeated candidate from the Democrat Party and a former MP in the same constituency, has vowed to submit evidence of what he claimed to be poll fraud in Sunday's elections to support a probe by the regulator.