SINGAPORE - Mr Chen Show Mao, 59, was heralded as the Workers' Party's (WP) star catch when he was unveiled as a candidate in the 2011 general election.

A high-flying international corporate lawyer and a former government scholar with degrees from Harvard, Oxford and Stanford, Mr Chen had all the hallmarks of a candidate more typically associated with the ruling People's Action Party (PAP).

His entry into politics was hailed as a high-water mark for the opposition and a demonstration that the PAP was not the only party in town that could attract candidates of a high calibre.

In his electoral debut in 2011, Mr Chen was part of the WP's A team led by then secretary-general Low Thia Khiang, now 63, who had left the safety of his Hougang SMC stronghold to mount an ultimately successful bid for Aljunied GRC.

It was the first time an opposition party had won a GRC.

Five years later, in May 2016, Mr Chen made an unprecedented move of his own, taking on Mr Low for the post of WP leader in the party's central executive committee elections. Mr Low had never faced such a challenge since he took over the party in 2001.

This came just months after the WP narrowly retained Aljunied GRC with 50.96 per cent of the votes in the 2015 general election.

Mr Chen's attempt was unsuccessful and Mr Low fended off the challenge with 61 votes to 45. Three months later, Mr Chen resigned as party treasurer but remained a committee member.

He also did not challenge his fellow Aljunied GRC MP and then assistant secretary-general Pritam Singh for the top spot in the party's next election in 2018.

During a press conference on Thursday (June 25), Mr Singh announced that Mr Chen, Mr Low and Hougang SMC MP Png Eng Huat will not be standing as candidates in the coming general election.

Mr Singh emphasised that this was a collective decision reached by the party leadership and the three MPs some time ago.

He said all three had expressed a desire to step down at the end of the current term of Parliament, though he added that this was not a retirement.

Non-constituency MP Leon Perera, 49, and former NCMP Gerald Giam, 42, will step in for Mr Low and Mr Chen in the WP's Aljunied GRC team, Mr Singh said.

Mr Png, 58, a businessman and card-carrying member of the WP since 2006, also entered politics in 2011, when he was fielded as a candidate in East Coast GRC.

His team lost by a nine-point margin to the PAP team led by then National Trades Union Congress secretary-general Lim Swee Say.

But a year later, Mr Png strode into the national spotlight when he was picked as the WP's candidate to succeed WP MP Yaw Shin Leong, who left under a cloud, in the party's Hougang base during the 2012 by-election.

Beating the PAP's Desmond Choo with 62.1 per cent of the vote, Mr Png remained as MP for the ward for eight years until Parliament was dissolved on Tuesday (June 23).

He defended the WP stronghold again in 2015 but won with a smaller vote share of 57.66 per cent against PAP newcomer Lee Hong Chuang.

Picking up the baton in Hougang is NCMP Dennis Tan, 49, who contested Fengshan SMC in the previous election.

Mr Tan, a shipping lawyer, will look to defend what has been the longest-held opposition ward, which Mr Low won in 1991.