Isarapong raked in 49,014 votes in Chumphon’s Constituency 1, with Palang Pracharath’s Chavalit Ajharn trailing with 32,281 ballots.
Meanwhile in Songkhla’s Constituency 6, Supaporn pulled in 40,963 votes with Palang Pracharath’s Anukool Pruksanusak coming in second with 35,282.
The by-elections were held to replace Democrat veterans Chumpol Julsai and Thaworn Senneam, who were stripped of their MP status over their role in protests that led to the 2014 coup.
Democrat leader Jurin Laksanawisit, who is also commerce minister, thanked voters via Facebook on Sunday, saying: “As party leader, I vow that the party will continue working hard for the people in Chumphon, Songkhla, the South and across Thailand under the party’s ideology. We will make sure that we deserve your trust.”