The by-election in Nakhon Pathom's Constituency 5 saw the race between the Future Forward Party (FFP) and its main rival Chartthaipattana go down to the wire on Wednesday and FFP lost the seat.
The unofficial result was known late last night. Tallies showed FFP candidate Pairatchote Chantarakhachorn losing the seat to Padermchai Sasomsap of the Chartthaipattana Party.
Mr Padermchai received 37,675 votes, Mr Pairatchote had 28,216 votes followed by Democrat Surachai Anuttato on the third place with 18,425 votes.
At 7.30pm on Wednesday, Mr Padermchai thanked his supporters and said he was happy he had given them hope, as he was leading according to the finished vote counts at 160 out of 193 polling stations.
The contest was organised after FFP MP Jumpita Chankachorn stepped down as a result of injuries sustained in an auto accident during the election campaign.
Ms Jumpita won the seat in the general election with 34,164 votes.
Seven political parties took part in the poll, with voting closing at 5pm.
Mr Pairatchote, who is also Ms Jumpita's husband, was the strong favourite but Mr Padermchai's performance in the race came as a surprise to some political observers. He came fourth with 12,279 votes in the March poll.
In the early stage of the election campaign, the contest looked set to be a three-horse race between the FFP, the Democrat Party and Chartthaipattana.
The Palang Pracharath Party (PPRP) did not compete in the by-election but urged its two partners in the governing coalition, Chartthaipattana and the Democrats, to do their best to win the seat for the government camp.
Riding on the FFP's popularity, Mr Pairatchote emerged as the favourite to retain the House seat for the party and the opposition camp.
The Pheu Thai Party did not field a candidate.
His election campaign saw key FFP figures in attendance -- party leader Thanathorn Juangroongruangkit, secretary-general Piyabutr Saengkanokkul and spokeswoman Pannika Wanich.
Because the poll was scheduled mid-week with many voters going to work, the number of voters could be lower than expected, which might result in the FFP capturing fewer votes than last time, Mr Piyabutr said.
However, Stitorn Thananithichote, acting director of the Office of Innovation for Democracy under the King Prajadhipok Institute, said the popularity of the FFP is in question as the party has been involved in many political disputes and has yet to show outstanding achievements in its role on the opposition benches over the past seven months.
The PPRP and the Democrat Party are widely thought to share the same political base in the constituency and with the PPRP out of the contest, its supporters were expected to pick Democrat candidate Surachai, who came second in March.
According to political observers, the gap between the winner and the runner-up could narrow significantly this time round.
In the March election, Ms Jumpita won with 34,164 votes. Mr Surachai came second with 18,970 votes, followed by Rawang Netphokaew of the PPRP (18,741 votes) and Mr Padermchai (12,279 votes).