The opposition here used to be able to rely on three "P"s for support during an election period.

These are: Protest votes, arising from dissatisfaction among those in the electorate who have been unhappy over government policies;

Votes from those who Pity, sympathise and see opposition parties as underdogs who need their backing for taking on the might of the ruling People's Action Party (PAP); and

Votes of Passion - from those who are true believers and share in the opposition's cause to serve as a check on the Government and, at some point down the road, to become that government.

But going forward, the opposition parties may have to focus more on the last "P" to avoid the big swing in the national vote share and also the winning margins in almost all the 29 battlegrounds towards the PAP at the Sept 11 General Election.

It could be the biggest lesson for opposition parties as they take stock and pick up the pieces after a surprisingly heavy defeat by the PAP at the ballot box.

The voting results showed that all parties, including the Workers' Party (WP) and the Singapore Democratic Party (SDP), have suffered from and will continue to be hit by a decline in Protest and Pity votes.

It is either as a result of their own will or the PAP's actions.

In the past, opposition parties could count on a minimum level of support by Protest voters who believe they have suffered from government policies or perceive the PAP to be out-of-touch with the masses, or placing economic growth at all costs above the people's welfare.

They cast their votes not so much to support the opposition but in pique and anger at the PAP.

But since the 2011 polls which saw a historic loss by the PAP to the WP in Aljunied GRC, policy tweaks by the Government in key areas such as housing and immigration have managed to quell discontent to a large extent.

It is hard to imagine that the PAP - buoyed by its 69.9 per cent vote share at the polls - its best performance since the 2001 election and a 9.8-point surge from the 60.1 per cent it received in 2011 - would stop doing what has clearly worked for it at the 2015 elections.

As for the "Pity" votes, opposition parties could once count on a substantial level of sympathetic supporters who want to reward them for their willingness to take on the PAP, come what may.

Many also wanted to help ensure that the party and its candidates do not lose their electoral deposits.

But such sympathetic sentiments have dwindled now, given changes in the political climate here which have resulted in more people willing to come out, join an opposition party, and stand as candidates against the PAP.

At the 2011 polls, 82 out of the 87 parliamentary seats were contested, marking a record high since the 1972 polls.

The election on Friday had contests in all 89 seats, a development not seen since Singapore's independence in 1965.

With seemingly no lack of opposition candidates in the pipeline, the electorate is no longer as moved by the need to cast the Pity vote just so the opposition can stay in the game .

For the more established parties, like the WP, such votes have been on the wane, especially as voters recognise that the party has been able to attract more high-calibre candidates to its ranks in recent years. Under such circumstances, it is also disingenuous of opposition parties to expect voters to see them as weak and in need of support.

The decline among those who would cast the Protest and Pity votes, coupled with an insufficient rise in the number of Passion votes, goes some way to explain the PAP's handsome victories of above 70 per cent of the vote share in six out of 13 single-seat wards, and nine out of 16 group representation constituencies (GRCs).

The lesson is clear: Opposition parties must start building up a critical mass of Passion voters.

But this is the toughest type of voter to cultivate and satisfy.

One just has to look at the WP's progress over the past decade, and its setback at the latest election, to understand why.

Since 2001, when Mr Low Thia Khiang took over the reins as secretary-general, the WP has been trying to attract Passion votes from Singaporeans who seek a "rational, responsible and respectable" opposition party capable of being a check on the Government by having at least one-third of the MPs in Parliament. That's the minimum required to prevent changes to the Constitution.

The WP still does tap the Protest and Pity vote by pointing out what it deems to be unfair treatment or bullying by the PAP. But its priority has been to have a large core of voters who believe in its cause.

For this approach to succeed, the WP needs voters to believe in the need for an opposition party to play the role of watchdog on government - and to believe even more, that the WP is the party capable of playing that role well.

The WP's performance at the Sept 11 polls - losing Punggol East, narrowly retaining Aljunied GRC, seeing its margin cut in its stronghold of Hougang, and failing to capture East Coast GRC and Fengshan - shows that it does not have enough of such voters yet.

Why so? It may be that not enough people are convinced of the need for a check on the Government. And among those who believe in the need for such checks, the question some might have is whether the WP has the ability to take on such a role.

There is little doubt the ongoing dispute between the WP and the Government over financial lapses at the Aljunied-Hougang-Punggol East Town Council has contributed to voters wondering if the party has what it takes.

Also, there are those who ask if the WP is trying to grow too big too fast, and whether it has become too cocky after its recent successes in winning a GRC in 2011 and two by-elections thereafter.

The WP's 2015 campaign began conservatively with Fengshan and East Coast appearing to be its main targets. But after the mid-point, it moved up a gear and launched an offensive to also snag Marine Parade GRC helmed by Emeritus Senior Minister Goh Chok Tong.

Its criticisms at a rally last Monday of "bad policies" during Mr Goh's 14 years as prime minister appeared to have triggered concerns among voters and brought to the fore the reality that it was intent on ousting a popular and former prime minister. The party would have come across as being overambitious without having proved convincingly that it had the ability to take on bigger responsibilities, such as overseeing additional town councils.

A dash for three GRCs at one go - when questions had been raised about its ability to manage just one - may have contributed to its undoing at the polls.

Grooming Passion voters will require the WP to exercise more finesse in the messaging of its goals and to move towards its political goals at a pace at which voters are comfortable with.

For the SDP, it is a similar story as voters appear to still have reservations over whether the party and its chief, Dr Chee Soon Juan, have truly shed their previously combative style for a more constructive path.

But it is not the end of the world for the opposition. The inability to make further gains on what was achieved in 2011 suggests that more effort needs to be made to convince and grow the Passion voter pool.

If they fail to do so, then opposition parties and politics here will continue to be subject to the vagaries of swings in the mood and perception of the electorate. And voters will regard the opposition as useful only when they want to send signals of unhappiness to the PAP.