Changes to the voting system, introduced by the
2017 constitution, were first implemented in this election. Whereas previous general elections in Thailand since 2001 used a
parallel voting system, with voters marking two ballots, one for their constituency and one for a nationwide party list, the new system, referred to as "mixed-member apportionment", uses a
mixed single vote for both the constituency and the party list (which makes it a variant of the
mixed-member proportional system). The 350 constituency seats are won by
first-past-the-post voting as in previous elections. However, the 150 party-list seats serve a
compensatory function, and are allocated so as to give each party a total number of seats proportional to the nationwide number of votes they received (top-up). Other changes introduced by the new constitution are the removal of the requirement that the prime minister be selected from among the elected members of the House of Representatives. Instead, each party may name, in advance, up to three candidates to nominate as prime minister if they achieve a majority. Also, the prime minister will be voted on by the combined
National Assembly, which also includes the 250 members of the
Senate, Thailand's upper house, rather than only by the House of Representatives, as was previously done. As the entire sitting Senate will be appointed by the NCPO, commentators say that the change opens up the strong possibility that Prayut will become Thailand's next prime minister despite pro-Prayut parties being predicted to win fewer seats in the House of Representatives.
Constituency boundary controversy Under the new constitution, the number of constituencies was reduced from 400 to 350. In 2018, the
Election Commission (ECT) was tasked with drawing up new district boundaries. However, shortly before the ECT were about to announce the new boundaries, Prime Minister Prayut invoked
Section 44 of the constitution to issue an order pushing back the deadline and delaying the announcement. The order also exempted the ECT from complying with existing districting laws and allowing them to draw constituency boundaries in any manner they wished. The move sparked outrage from the
Pheu Thai and
Democrat parties and the watchdog organisation Open Forum for Democracy Foundation. They argued the purpose of the delay was to enable the ECT to draw constituency boundaries that favoured Prayut's
Palang Pracharath Party. Some journalists and commentators compared this to
gerrymandering, while others opined that the changes meant that the elections were a foregone conclusion. Election Commission chairman Ittiporn Boonpracong denied the allegations, citing his eye surgery as the cause of the delay. The EC completed and released the new district constituencies on 29 November. Following the release, political parties and watchdog organisations stated that they had found several instances of gerrymandering that would benefit the Palang Pracharat Party. ==Campaign==