The Bukit Batok by-election to fill the seat vacated by Mr David Ong could be held as early as May, soon after Parliament completes its debate on the Budget on April 15.
But observers doubt the People's Action Party (PAP) will be in such a rush, even as Prime Minister Lee Hsien Loong said he will call for one "in due course".
Institute of Policy Studies deputy director for research Gillian Koh said the ruling party "may even take some time to ensure that no local issues pop up to make things more difficult for them".
National University of Singapore political scientist Bilveer Singh said this "classic case of moral deficit" - Mr Ong resigned citing an affair - comes at a bad time "for a party that talks so much about ethics, cleanliness, uprightness, leadership".
He expects the PAP to allow some "cooling down" time first.
Although Mr Ong won 73 per cent of the vote in last year's general election, this by-election will likely see a closer fight, said observers.
The single seat has not always been a safe bet for the ruling party.
Prior to GE2015, the last time Bukit Batok was contested as a single seat, in 1991, the margin was slimmer. PAP candidate Ong Chit Chung defended his seat with 52 per cent of the vote against the SDP's Kwan Yue Keng.
Ahead of GE1997, the seat was absorbed into Bukit Timah GRC, and later became part of Jurong GRC.
Dr Ong died in 2008, sparking a debate on whether a by-election had to be held. As the ward was part of Jurong GRC, the conclusion was that there was no need for one.
This time, there will be one, and the SDP intends to contest it.
The likely result will depend on who is fielded and how many players enter the fray, said Associate Professor Eugene Tan of the Singapore Management University. Of the SDP's choice, he said: "I think it will boil down either to Dr Paul Tambyah or Dr Chee Soon Juan."
Both were in the SDP's four-man team in Holland-Bukit Timah GRC in GE2015, where the SDP had its best showing with 33 per cent.
Political observer Derek da Cunha said on Facebook the by-election is a chance for Dr Chee to aim for Parliament, but said "it will be more of a referendum on his relative popularity instead of anything else".
Experts noted that Bukit Batok's western location puts it outside traditional Workers' Party territory. WP did not say what its plans were.
People's Power Party leader Goh Meng Seng said he will discuss with the SDP whether to contest. The Singapore Democratic Alliance, National Solidarity Party and Reform Party all said they had yet to decide.
Said Dr Singh: "I don't think the PAP is in danger of losing the seat."
Prof Tan said the upcoming by-election will be a "very different scenario" from the 2013 Punggol East by-election, which PAP lost to WP candidate Lee Li Lian amid residual anti-PAP sentiment from GE2011. Dr Koh said Ms Lee was helped by unhappiness over local issues such as incomplete renovations of a wet market and shopping mall and a lack of childcare.
Public housing accounts for 95.7 per cent of Bukit Batok households, with four-room flats the most common. Voters will "be mindful" of the fact that their ward will no longer come under Jurong-Clementi Town Council if the PAP lose, said Prof Tan. The SDP has not run a town council since the 1990s.
Bukit Batok's history as part of Jurong GRC could be to the PAP's advantage, said Dr Koh. "Perhaps the PAP can ride some way on the strength of political capital that Deputy Prime Minister Tharman Shanmugaratnam and Speaker Madam Halimah (Yacob) have built up over the years in that area," she said.
The past two by-elections:
The last two by-elections were both prompted by MPs stepping down due to personal indiscretions.
Late January, 2012: Rumours of an extramarital affair between first-term Workers' Party MP Yaw Shin Leong and a fellow WP member begin to spread online.
Feb 14: WP expels Mr Yaw, who had won the Hougang seat in the 2011 General Election, rendering the seat vacant.
May 9: Writ of election issued.
May 16: Nomination Day May 26: By-election held. WP candidate Png Eng Huat wins with 62.1 per cent of the vote against PAP's Desmond Choo.
Dec 12, 2012: People's Action Party MP Michael Palmer, who was then Speaker of Parliament, resigns after admitting to an extramarital affair with a People's Association constituency director.
Jan 9, 2013: Writ of election issued. Jan 16: Nomination Day Jan 26: By-election held. In a four-way fight, WP candidate Lee Li Lian wins with 54.5 per cent of the vote.
PAP candidate Koh Poh Koon garners 43.7 per cent of the vote, while Reform Party candidate Kenneth Jeyaretnam receives 1.2 per cent and Singapore Democratic Alliance candidate Desmond Lim gets 0.6 per cent.
This article was first published on March 13, 2016.
Get a copy of The Straits Times or go to straitstimes.com for more stories.