In the Central Plains where popularity plays a crucial role in winning elections, a tight race is likely between the Democrats and the Palang Pracharath Party (PPRP) with the Bhumjaithai Party also potentially claiming a sizeable share of seats in the region, according to political observers.
The Central Plains is the second-largest region in terms of constituencies, after the Northeast, with 92 MP seats up for grabs.
A source at Bhumjaithai said although Pheu Thai dominated the region in the last election, it would be a different story this time as many well-known members have defected to other camps.
Members have also left Chartthaipattana, leaving only Suphan Buri and Nakhon Pathom as a sure bet for the party, according to the source.
With Chartthaipattana expected to struggle to maintain support in some provinces, the Democrats, PPRP and Bhumjaithai could be the major competitors in the region.
The source added that parties which can tap into the "undecided" pool, which currently accounts for 40-50% of the region's eligible voters, would have a chance to win big there.
In this region, the source said, a candidate's popularity, rather than their party's image, woos voters.
"In the Central Plains, I believe popularity carries more weight. Those who switch parties face boycotts from voters as long as they visit their constituencies regularly," the source said.
The Democrats, PPRP and Bhumjaithai are predicted to be in for a close battle, each winning a similar number of votes. Chartthaipattana is also expected to take some seats, especially in its strongholds.
Meanwhile, the source said Bhumjaithai is likely to prevail in Prachin Buri.
Even in Chartthaipattana's stronghold of Suphan Buri, Bhumjaithai could have a chance to make a substantial gain after veteran politician Jongchai Thiengtham switched from Chartthaipattana to the camp, the source said.
Overall, it is estimated the Democrats and the PPRP are likely to capture at least 20 seats each in the region, the source added.
Chartthaipattana director Nikorn Chamnong admitted his party's MP tally in the Central Plains could shrink from previous elections. But he voiced confidence the party will manage a clean sweep in Suphan Buri.
The party also has a chance to win seats in Phetchaburi, Nakhon Pathom and Samut Sakhon, he said.
"In the Central Plains, I have to say the party's popularity is not the issue," said Mr Nikorn.
He said, in general, the Democrats and PPRP will lock horns in many constituencies in the region. In Chai Nat, the PPRP gained traction after Anucha Nakasai, a former MP of the defunct Thai Rak Thai Party who still wields much political influence, signed on as a member.
Pheu Thai, according to Mr Nikorn, did not field a lot of candidates in the region as the party appears more keen to secure the North and Northeast.
Despite the odds, Chartthaipattana could beat the Democrats and the PPRP in some constituencies. "Bhumjaithai also looks fearful as they are vigorously campaigning for votes. They are likely to gain votes that would be counted for party-list MPs," said Mr Nikorn.
Yutthaporn Isarachai, a political scientist from Sukhothai Thammathirat Open University, noted Chartthaipattana no longer dominates the Central Plains the way it did when Banharn Silpa-archa, the party's patriarch, was leader.
Bhumjaithai is likely to emerge as a contender to be reckoned with after it pulled in former Chartthaipattana and Pheu Thai members, the lecturer said, adding that the PPRP got a boost after it welcomed the Phalang Chon political group into its fold.
The group commands swathes of Chon Buri where the Democrats also competed fiercely in previous polls.
The PPRP will have to compete with the Democrats, who also retain firm footholds after winning many seats in Rayong, Chantaburi and Trat in the past, he said.
In Mr Yutthaporn's view, Bhumjaithai will give Chartthaipattana a run for its money. Chartthaipattana may be in for a huge election setback after it lost the Thiengtham family to Bhumjaithai.
It may be the first time Chartthaipattana cannot defend its home ground.
In the upper Central Plains provinces, such as Chai Nat and Nakhon Sawan, it is possible that the PPRP will be dominant, according to Mr Yutthaporn.
The PPRP, he said, could also pick up some seats in Lop Buri after it welcomed a group of experienced politicians led by Amnuay Klangpha, a former Pheu Thai MP, into its fold.
Mr Yutthaporn agreed that voters in the Central Plains tend to have an attachment to candidates, not parties, judging from the ability of several political families to get re-elected despite their frequent defections.
Political families are common in the region, and include the well-known Silpa-archas in Suphan Buri, Klangphas in Lop Buri and Nakasais in Chai Nat as well as the Khunpluems in Chon Buri.