Hougang, Punggol East, and now, Bukit Batok - Singaporeans will witness a third by-election in just under four years. All three arose after resignations by the incumbent MPs over alleged extramarital affairs.

The first two polls were won by the opposition, helped by what politicians and analysts term the "by-election effect". This refers to voters choosing the opposition party, knowing that the ruling People's Action Party (PAP) will not be toppled from power overnight.

But how big this effect is depends on the size of the middle ground of swing voters, who are not loyal to any party and may change their vote from election to election.

In a by-election, this group may vote for the opposition - who may be regarded as the underdog - in a desire for more political plurality.

But this time, it is not so clear whether this "by-election effect" will have a significant influence.

The Bukit Batok by-election is very different from the two most recent local polls, analysts tell The Sunday Times, citing several factors that may well blunt any leaning to buoy up the opposition.

The upcoming poll pits the PAP candidate, lawyer Murali Pillai, 48, against Singapore Democratic Party (SDP) chief Chee Soon Juan, 53.

Factors at play include qualities of the candidates and the party they represent, and the needs and demographic composition of the constituency.

Institute of Policy Studies deputy director for research Gillian Koh expects a relatively even match between Mr Murali, with 16 years of grassroots experience in Bukit Batok, and Dr Chee, whom "people identify as a leader of the opposition movement and voters may think of as an 'underdog'."

The Sunday Times examines the circumstances of the May 7 Bukit Batok poll, and those of Hougang in 2012 and Punggol East in 2013.

OPPOSITION'S PREVIOUS SHOWING

The last two by-elections were won by the Workers' Party (WP), which capitalised on simmering frustrations over purported policy missteps by the Government that had resulted in high living costs and a crowded public transport system.

The momentum was also with the WP, which at GE2011 made inroads into Parliament, ousting two Cabinet ministers to win Aljunied GRC - the first group representation constituency to fall. The impetus remained with the WP going into Hougang in 2012 and Punggol East in 2013, as the party won these and demonstrated this much-vaunted by-election effect.

In Hougang at GE2011, WP candidate Yaw Shin Leong won with 64.8 per cent of the vote against Mr Desmond Choo of the PAP.

Despite a scandal that erupted after Mr Yaw went missing in action in 2012, its by-election candidate Png Eng Huat still managed to retain the seat with 62.1 per cent of the vote. This was a slight dip of 2.7 percentage points in the vote share.

Hougang has been a WP stronghold since its chief Low Thia Khiang prised the seat from the PAP in 1991. And despite his move to contest Aljunied GRC in 2011, voters continued to show their support for Mr Low's "personal brand".

In Punggol East in GE2011, PAP candidate Michael Palmer won with 54.5 per cent of the vote in a three-way fight. His opponents were the WP's Lee Li Lian (41 per cent) and the Singapore Democratic Alliance's Desmond Lim (4.45 per cent).

By the time of the by-election 1 ½ years later, a number of national issues were still unresolved. There was also a host of unresolved local issues - from stalled upgrading works at Rivervale Plaza to a lack of coffee shops in the new neighbourhood. The PAP was also criticised for "parachuting" its candidate in.

Ms Lee, with her continued presence as a candidate in the 2013 by-election, thus benefited from a groundswell of desire for more opposition voices in Parliament.

She scored 54.5 per cent of the vote in a four-way fight, an improvement of 13.5 percentage points on her GE2011 showing.

Her opponents were the PAP's Dr Koh Poh Koon (43.7 per cent), the Reform Party's Kenneth Jeyaretnam (1.2 per cent) and the SDA's Desmond Lim (0.6 per cent).

WHO HAS THE MOMENTUM NOW?

The May 7 poll in Bukit Batok comes a mere eight months after GE2015, when the PAP's David Ong, 54, won 73 per cent of the vote in a three-way fight.

His two opponents were the SDP's Sadasivam Veriyah (26.4 per cent) and independent candidate Samir Salim Neji (0.6 per cent).

This means Dr Chee, who was part of a four-man SDP team that contested Holland-Bukit Timah GRC last year, needs a swing of at least 23.7 percentage points to beat Mr Murali. This also means that - all things being equal - Dr Chee needs to convince about 6,000 residents who voted for the PAP eight months ago to do otherwise.

The poll occurs at a time when momentum appears to be with the PAP, which just eight months ago rode on a wave of positive sentiment to win 69.9 per cent of the national vote share in GE2015.

The WP's Ms Lee lost her Punggol East seat and the WP held on to Aljunied GRC by a thin margin, its lapses in town council management taking centre stage at the hustings.

Some factors contributing to the PAP's victory, analysts have said, were the feel-good effect of the SG50 celebrations and sentiment over the death of founding prime minister Lee Kuan Yew.

Singaporeans, too, largely recognised the efforts of a government that recently has been open to recalibrating policies, and tried to re-establish rapport with its people.

Further, the by-election follows the Budget in March which people responded "quite positively to", Singapore Management University (SMU) law don Eugene Tan notes.

"In terms of the overall mood of the voters, it's actually quite different as compared to the last two by-elections," he adds.

This might be why the major issue that has emerged in the by-election thus far has been local: neighbourhood upgrading plans.

Last Sunday, Mr Murali unveiled $1.9 million worth of infrastructure plans for a precinct in the mature town where nearly 80 per cent of HDB flats are at least 30 years old.

Dr Chee retorted that linkways should "not be made into an election issue", and called such estate upgrading plans a "knee-jerk reaction" to entice voters.

Mr Murali said he can only push through the plans if he is MP and Bukit Batok remains part of the larger Jurong-Clementi Town Council.

WHICH PARTY HAS FAMILIARITY?

In the case of the 2013 by-election, the proximity of Punggol East to the WP-held Aljunied GRC and Hougang could have had a spillover effect on voters there, analysts said.

National University of Singapore (NUS) political scientist Reuben Wong notes that while the WP has been active in north-east Singapore, the SDP "has not been very present in Bukit Batok" consistently.

The SDP's previous forays in the constituency were in 1988 and 1991 when its candidate Kwan Yue Keng contested there, but lost both times to the PAP's Dr Ong Chit Chung.

It contested in adjacent Bukit Gombak, also in 1991, when lawyer Ling How Doong won against the PAP's Dr Seet Ai Mee with 51.4 per cent of the vote. But he then lost in 1997 with 28.4 per cent of the vote.

Bukit Batok was drawn into Bukit Timah GRC and then Jurong GRC at subsequent polls, and was resurrected as a single seat only last year.

Bukit Timah GRC went uncontested in GE1997, and likewise Jurong GRC in GE2006.

In GE2001, a five-man SDP team that included Dr Chee won 20.25 per cent of the vote in Jurong GRC. In GE2011, the GRC was contested by the National Solidarity Party.

"The SDP does not have that kind of ground experience and familiarity in Bukit Batok like the WP did in Punggol East," says Associate Professor Wong.

Another factor that swung the pendulum to the WP in the 2013 Punggol East poll was the demographic profile of the area, with its many young and likely more liberal voters. At the time, there were no three-room flats and about 60 per cent of residents lived in five-room flats, executive flats or private housing. Only a quarter of its residents were more than 50 years old.

A large part of this middle- to upper-middle-income group of residents may have wanted greater opposition in Parliament and, with the "by-election effect", felt more inclined to take risks.

Yet in Bukit Batok today, a good 70.8 per cent of public housing are three- or four-room flats. And over 30 per cent of the residents are above 50 years old. They may need to hear more from Dr Chee, who is trying to shake off the image of a combative politician who was previously sued for defamation.

Says Associate Professor Tan of SMU, a former Nominated MP: "I don't think voters here are going to make too much of Dr Chee being a long-time politician."

He adds: "Voters will scrutinise the candidates on who they are, what they represent and what sort of confidence they inspire."