Former Democrat Party leader Chuan Leekpai warned his party it would tread a dangerous path if it joined a coalition government full of people who he said listen to so-called cheats.
Mr Chuan said on Thursday that if certain parties continued to be attached to cheats and believed in them, the Democrats would find themselves in dangerous water by being associated with them.
However, the Democrat patriarch insisted his remark was hypothetical.
Mr Chuan made his comments as the Move Forward Party (MFP) tries to form a government with seven other parties in the so-called pro-democracy alliance, which includes the Pheu Thai Party.
The Democrats are also convening an assembly on Sunday to elect a new executive board and a new leader to replace Jurin Laksanawisit, who resigned earlier in a show of responsibility over the party's poll defeat.
Mr Chuan said the Democrats should keep their ambitions in check if and when it decides whether to take part in a government.
It was speculated that if the MFP-led eight-party alliance, which does not include the Democrats, failed to assemble a government, a new line-up may be established with the Democrats invited to join.
Mr Chuan reminded the Democrats it only had 25 MPs, including three from the party list. "We even lost on our own turf in the South to Prime Minister Prayut Chan-o-cha," he said.
Gen Prayut is also the chief strategist of the United Thai Nation Party.
"We must realise how small we've become. Only when we face reality will it dawn on us what kind of people we need to sort out [the party's] problems," Mr Chuan said. He added that throughout its 77 years, the party has firmly upheld its zero tolerance for voting fraud. That should continue to be the party's mantra, and future party leaders should adhere to it.
Mr Chuan said the new Democrat leader should unite and restore the party to its former glory. He said whether the new party executives were from younger or older generations made no difference.
The new executive board should be a mixture of people who represent the past and future, he said.
Mr Chuan, also a former premier, admitted he was worried some party members were overlooking the importance of denouncing corruption.
So far, Alongkorn Ponlaboot, the acting Democrat deputy leader, is the only member who has announced his bid to run for the party leadership.
Other potential candidates include Suchatvee Suwansawat, the Democrat Party's Bangkok governor candidate; Watanya Bunnag, who heads the party's working group on political innovations; and Det-it Khaothong, the Democrats' acting secretary-general.
