The by-election in Rayong’s constituency 3 follows the resignation of Move Forward MP Nakhonchai Khunnarong after he confessed that he had been jailed 24 years ago for theft.
The Constitution stipulates that MP candidates must never have served a jail sentence.
The Election Commission set September 10 as the date for the by-election.
Speaking in the rain, Pita told an enthusiastic crowd of supporters who had gathered to greet him in the province's Khao Chamao district that his party stood by its pledge not to align with junta-backed parties.
He added that before the election, certain parties had vowed not to join forces with coup-supporters, but after receiving people's votes, they had gone ahead and allied with them.
“Will you [the people] tolerate that?” he asked.
The Pheu Thai Party has ditched Move Forward from the coalition and is now seeking to form a government with conservative parties that grew out of the post-coup junta.
Pita said that despite being compelled to serve in opposition, Move Forward would be an opposition that represents the people.
He also urged supporters to give Move Forward’s Rayong by-election candidate Phongsathorn Sornphetnarin a landslide victory. Phongsathorn said the by-election was vital for everyone, not merely for Move Forward. It presented an opportunity to show that the sovereignty of the people is the supreme authority in the country, he explained.
Phongsathorn, 41, has been a strong advocate on local matters, including efforts to prevent factory construction deemed damaging to the area.
As a student, he served as secretary of the activist-led Student Federation of Thailand. Under the previous Parliament, he worked as a Move Forward MP's personal assistant.