In last Friday's general election, the Workers' Party (WP) made another breakthrough by wresting a second group representation constituency from the People's Action Party (PAP).
Opposition veteran Chiam See Tong had once described GRCs as "impenetrable fortresses", and argued that the opposition had to win at least one to dispel this impression.
Political analysts say GE2020 has shown the PAP that there is no safe haven, even with an anchor minister helming the team.
But they added that the GRC system remains a difficult obstacle for opposition parties to surmount, despite the WP's electoral successes.
The WP made history in 2011 when it became the first opposition party to win a GRC, 23 years after the system was first established.
At the recent election, the party won the new Sengkang GRC with 52.12 per cent of the votes, and widened its vote share in Aljunied to 59.95 per cent.
In a commentary for The Straits Times, NUS economics professor Ivan Png said regression estimates that he ran showed the PAP did relatively better in single-member constituencies than in GRCs. In GE2020, the PAP vote share in five-member GRCs was 10.9 percentage points lower than in SMCs.
In a Facebook post on Sunday, former PAP MP Inderjit Singh said the ruling party did not perform up to par in GRCs where opposition parties fielded strong slates.
"It is clear that the GRC system is no longer an advantage for the PAP," Mr Singh wrote, adding that the loss of a second GRC and three political office-holders - including labour chief Ng Chee Meng - is a "disaster" for the PAP.
"If the PAP fails to reform, this may be the beginning of a downward spiral which will threaten the future of the party," he said.
NO 'MAGIC' IN GRCS?
Associate Professor Eugene Tan, a law don at the Singapore Management University, said the WP's success in Aljunied and Sengkang has shown that GRCs are not impregnable fortresses for the PAP.
"There is no magic in a minister helming a GRC, making it invincible," he said. "The PAP may well win big in GRCs, but it must not be forgotten that it has lost, and can lose big, in GRCs."
For opposition parties, one longstanding challenge has been attracting enough good candidates. In past elections, parties would typically manage to field only one or two star candidates in each GRC.
This time, the WP's Sengkang slate was as good as - or close to - that of its opponents, Mr Singh told ST. The Progress Singapore Party (PSP) team in West Coast also offered "better quality and diversity" in comparison with their PAP counterparts, he added.
"The scales will be tipped the next round, as more good candidates are willing to join the opposition," Mr Singh said.
But observers also cautioned against overstating the degree to which GRCs are now vulnerable, even to opposition parties that field credible teams.
Political analyst Derek da Cunha noted that the PSP unveiled a large contingent of 24 candidates in its maiden election, likely believing it could win several constituencies.
But its failure to take West Coast GRC - despite fielding party chief and former PAP MP Tan Cheng Bock as well as having Mr Lee Hsien Yang, the younger brother of Prime Minister Lee Hsien Loong, canvass for votes - illustrates the extent to which GRCs still remain PAP strongholds, he said.
National University of Singapore political scientist Bilveer Singh made the point that the WP's success can be attributed to how it has been working the ground in Aljunied for nine years.
Meanwhile, the younger voters of Sengkang likely voted against the PAP out of a distaste for its "punitive-type" politics, he said, referring to police reports made against the WP's Ms Raeesah Khan, and correction orders issued under the Protection from Online Falsehoods and Manipulation Act during the hustings.
Even so, the PAP still has influential ministers, the power of incumbency, and national resources, Dr Singh said.
"The WP is far from challenging the PAP except on two counts - if the PAP is split due to internal infighting, or the opposition builds a coalition - both of which seem highly unlikely in the near future."
Mr Harrison Cheng, associate director of consultancy Control Risks, noted that 15 out of the 17 GRCs are still in PAP hands. "The structural features of the GRC system ultimately mean that it is very difficult - though not impossible - to wrest control of a GRC," he said.
Dr da Cunha said: "In reality, the actual competition is still narrowed to between the PAP and the WP, and this is largely confined to the eastern part of the island."
When the GRC system was introduced in 1988 to ensure minority representation in Parliament, the polls that year saw Singapore divided into 42 SMCs and 13 three-member GRCs.
Three years later, the number of SMCs was halved and the size of GRCs increased to include four-member constituencies.
In 1996, a proposal to further increase the maximum size of GRCs from four to six members was tabled in Parliament.
All four opposition MPs in the House at the time cried foul.
Mr Chiam See Tong argued the move would help "new and weak" PAP candidates win seats by riding on the coat-tails of more experienced MPs, while WP MP Low Thia Khiang said the changes would make it even more difficult for opposition parties to win seats.
But then Prime Minister Goh Chok Tong said six-member GRCs were necessary to establish a "critical mass of residents" in some areas, so that community development councils could do their work.
The move to enlarge GRCs was politically neutral, as the party fielding the stronger team would naturally emerge victorious, he added.
PM Lee initiated reforms to reduce the average size of GRCs and create more SMCs ahead of GE2011 - a shift that has continued. In GE2020, there were 11 five-member GRCs, six four-member GRCs, and 14 SMCs.
How is the PAP likely to change its strategy in the next general election, given that it took a bruising this time round?
First, the party may re-evaluate what makes a strong team, especially given what Mr Cheng described as the "less-than-impressive" showing of several fourth-generation leaders who headed hotly contested GRCs.
He added that voters will pay close attention to the track record of the PAP's 4G leaders in the years leading up to the next GE, focusing in particular on how they engage the opposition, civil society and diverse voices outside Parliament.
"(This) will be a much stronger determinant of whether the public buys into the PAP's branding during the election campaign period as a sufficiently inclusive, responsive and unifying force in politics," he said.
The ruling party may also relook its strategy for GRCs, said Prof Tan.
One possibility, he said, is that the PAP could revive three-member GRCs and get junior political office-holders to helm them.
This would ensure minority representation while keeping the loss contained if the party loses, he added. It would also help manage the perception that the GRC system is stacked against the opposition.
Associate professor of political science Netina Tan of Canada's McMaster University expects the PAP to alter its candidate selection method to draw younger candidates.
The ruling party will also have to improve its online electoral strategies and timeliness to regain control of online narratives, she added.
She also flagged the possibility of boundary changes at the next election. Electoral boundaries have been known to change with elections, including in hotly contested seats .
Dr Singh said the PAP will have to "scan the ground" and adjust to what it finds, or risk more GRCs falling to the opposition. "I think the younger generation of voters want to see change in their political leaders, policies and in politics," he said.