Members of the People's Action Party (PAP) hold a spectrum of views on many issues, ranging from the conservative to the liberal, said Prime Minister Lee Hsien Loong, who is the party chief, yesterday.

For instance, some want to keep the Primary School Leaving Examination while others favour scrapping it, and some want to retain Section 377A, which criminalises sex between men, while others want to repeal it, he noted.

But regardless of their differences, all can be good party members, and it is important that the PAP can bring different groups and opinions together, he said.

The PAP's aim is to be a "broad tent", or a "broad church", PM Lee told the more than 3,000 members at a party conference as he urged them to keep Singaporeans united and encourage inclusive politics.

"We may not be able to reach a consensus on all issues, all the time.

"But we should always try to find common ground, and more importantly, maintain a shared space where the different views can be aired constructively, where we can engage in a way that does not erode trust and social cohesion," he said.

Failing to do so will result in the middle ground withering away. "The extremes will grow," he added.

Politics would become a "zero-sum game", organised along fault lines in Singapore society, and such politics would only make the fault lines deeper, he said. "People would be forced to take sides, you are either for me or against me."

Citing the United States, PM Lee said the politics is so deep that when families gather, they cannot talk politics. And it is reaching a point where marriages between Democrats and Republicans are frowned on.

"It is like between Romeo and Juliet - different clans, different tribes and different nations," he added.

In such a situation, it will be tough for the PAP, a centrist party, to hold everyone together and bring people from two ends together, he said.

Extremist protests, radical and populist groups will gain ground, and once society moves in this direction, it will be a downward spiral. It will be "practically impossible to rebuild centrist politics again and bring Singaporeans together again", he said.

Referencing a nursery rhyme, he said: "Humpty Dumpty sat on the wall... fell down, finished. It has happened to many countries, and it can happen to us, too."

PM Lee also noted that Singapore has done better than most in coping with stresses such as economic disruption and loss of identity.

This, he said, is not just a case of good luck but the result of deliberate choices and hard work over the generations.

Citing tripartism, he said it is a salient example of how Singapore brings people together. Although employers, unions and the Government have their own interests, they can work harmoniously because trust and confidence have been built up over many years.

"So when we talk about maintaining cohesion and centrist politics, it may sound ordinary and dull, but, in fact, it is vital for Singaporeans and very rare in the world.

"It is a key reason why we have done better than most," he added.