This was remembered as PAP's toughest election battle in its history and the campaign coincided with Singapore's joining of Malaysia on 16 September 1963. After two by-election defeats and two defections to UPP, PAP filed a no-confidence motion on 20 July 1961 and survived with a razor-thin 26 votes against 25 from the opposition and 13 PAP Assembly Members after wheeling in an ill PAP AM and former Singapore UMNO activist, Sahorah Ahmat, from the hospital to vote in its favour. The 13 dissidents were subsequently expelled and formed Barisan, a new party PAP alleged was a front for the Communist Party of Malaya. On 3 July 1962, with the National Referendum on merger with Malaysia on everyone's minds, AM Hoe Puay Choo resigned from PAP and joined Barisan on 11 August 1962. Five days later, UPP AM S. V. Lingam returned to PAP but in another five, Minister Ahmad Ibrahim passed away from liver cancer and PAP's majority in the legislature whittled back to zero. Barisan intended to field its iconic leader Lim Chin Siong in the vacated Sembawang seat should there be a by-election but Prime Minister Lee Kuan Yew decided not to hold one. Instead, he went for a fresh GE to court a new mandate and if PAP did not win, the Malaysian authorities could deal with a pro-communist ruling party. Before that, the PAP government launched "Operation Coldstore" on 2 February 1963 and detained several Barisan leaders, including Lim. Sole WP AM David Marshall resigned from the party he founded and stood as an independent. On nomination day, PAP again fielded a full slate and was challenged by three camps with enough candidates to also run for power. Apart from left-wing partners Barisan and PR that contested all except two seats, UPP surprisingly put up a large number and SA, a formalised umbrella of SPA, UMNO, MCA and MIC, had the endorsement of Malaysia's ruling Alliance Party but former Chief Minister Lim Yew Hock sat out. Barisan candidate for Sembawang turned out to be Chen Poh Chang, wife of Fong Swee Suan who was under detention. In the end, PAP not only emerged victorious but also regained its two-thirds majority and captured all wards held by Singapore UMNO, a result that displeased Malaysia PM Tunku Abdul Rahman. All except in three constituencies saw a record of 92 candidates forfeit their deposits. No independent candidates were successful, thus ending the presence of independent lawmakers. Two ministers who lost their seats, Kenneth Byrne and Tan Kia Gan, were later appointed chairmen of the Central Provident Fund and Housing and Development Board respectively. On 8 October 1963, three Barisan representatives were detained before they could be sworn in. They were S. T. Bani, Loh Miaw Gong and Lee Tee Tong. The PAP government also swiftly introduced an anti-hopping law to thwart future defections, which stipulated that lawmakers will lose their seats if they resign or are expelled from the parties that they were elected under. The PAP government swiftly introduced an anti-hopping law to thwart future defections, which stipulated that lawmakers will lose their seats if they resign or are expelled from the parties that they were elected under. Based on the outcome of the polls, 12 out of 15 Singapore seats in Malaysia's House of Representatives (Dewan Rakyat), the lower house of Malaysia's Parliament, was allocated to PAP. Nominated were PM Lee, Deputy PM Toh Chin Chye, ministers Goh Keng Swee, Ong Pang Boon, S. Rajaratnam, Yong Nyuk Lin, Jek Yeun Thong, Lim Kim San, Othman Wok and AMs Abdul Rahim Ishak, Wee Toon Boon and Ho See Beng. E. W. Barker later replaced Ho and was in turn replaced as Assembly Speaker by defeated SA AM, A. P. Rajah, who served for two years until 1966. Barisan took up the remaining three, namely by Chia Thye Poh, Lim Huan Boon and Kow Kee Seng. As for the Senate (Dewan Negara), Malaysia's upper house, PAP leaders accepted the suggestion of the Tunku to elect Singapore UMNO leader and unsuccessful SA candidate Ahmad Taff as one of the two Senators representing Singapore, with the other being Singapore Chinese Chamber of Commerce president Ko Teck Kin. The following year, PAP registered itself in the Peninsular to contest the Malaysia GE of 1964. Despite winning only one seat in Selangor, the move escalated PAP's differences with the Tunku and his Alliance leaders. On 2 September 1964, PAP was admitted into Socialist International as a member party.

Old Assembly dissolved: 3 September 1963 [Tue]
Nomination day: 12 September 1963 [Thu]
Polling day: 21 September 1963 [Sat]
New Assembly opening: 22 October 1963 [Tue]

Electorate: 617,650
Voter turnout: 587,433 (95.1%)

Election deposit: $500

Aljunied Geylang Serai Moulmein Southern Islands
Anson Geylang West Mountbatten Stamford
Bras Basah Havelock Nee Soon Tampines
Bukit Merah Hong Lim Pasir Panjang Tanglin
Bukit Panjang Jalan Besar Paya Lebar Tanjong Pagar
Bukit Timah Jalan Kayu Punggol Telok Ayer
Cairnhill Joo Chiat Queenstown Telok Blangah
Changi Jurong River Valley Thomson
Chua Chu Kang Kallang Rochore Tiong Bahru
Crawford Kampong Glam Sembawang Toa Payoh
Delta Kampong Kapor Sepoy Lines Ulu Pandan
Farrer Park Kampong Kembangan Serangoon Gardens Upper Serangoon
Geylang East Kreta Ayer Siglap
Seats: 51

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By-Elections November 1966
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Next: General Election 1968