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Two by-elections were held in 1961. The first by-election, for the Hong Lim constituency, was held on 29 April with the nomination day held on 11 March, while the second by-election, for the Anson constituency, was held on 15 July with the nomination day held on 10 June.
Background
On 27 July 1960, Ong Eng Guan was among the three PAP members to be expelled from the party for filing the famous "16 resolutions" in the Legislative Assembly against the government and challenged the PAP to defeat him there after his sacking from the Cabinet. Ong later resigned his seat in Hong Lim on 29 December and later expelled by the party after making open disputes with his Cabinet colleagues, including over the abolishment of the City Council when he was the last Mayor. Two other PAP members, S. V. Lingam and Ng Teng Kian, had followed suit but did not resign their seats.
On 20 April, nine days prior to the first by-election polling, another seat was vacated following the death of Anson's MP Baharuddin Mohammed Ariff. Workers' Party founder David Marshall, who was also a former Labour Front chief minister, would contest the seat.
Ong's landslide victory was attributed to his popularity with Hong Lim voters and his oratory skills. Incidentally, the PAP candidate Jek Yeun Thong was Ong's secretary during his time as Mayor in the City Council. Ong would form the United People's Party along with the two members on 18 June and would remain in his seat until his retirement in July 1965. However, following disagreements from the party, Lingam was later reinstated to PAP on 8 July 1962.
Marshall's victory marked the first WP presence in the legislature and the return to the Assembly since his resignation from the Labour Front in 1956; however, he lost his re-election bid in 1963. Anson would not elect another WP candidate again until two decades later in 1981, where Joshua Benjamin Jeyaretnam claimed the seat.
Two days after the Anson by-election, Prime Minister Lee Kuan Yew tendered his resignation and handed his role to the party's chairman Toh Chin Chye, who declined. Three days later, Lee then moved a motion of confidence in his own government; eight assembly members (AM) opposed the motion (which include David Marshall and members of the Singapore People's Alliance), while 16 abstained (which include the three members of Ong Eng Guan's UPP).
Lee then expelled the 13 PAP members, who perceived that these members who abstained the vote were allegedly pro-communist and disdain loyalty to his government; these members then formed the far left Barisan Sosialis (BS). The party reduced its majority of seats to one, and would lose its government majority on 3 July 1962, ahead of the year's integration referendum, where Ho Puay Choo defected to BS, and the death of Ahmad Ibrahim on 13 July.