Retired diplomat Bilahari Kausikan yesterday questioned Dr Thum Ping Tjin's loyalties by referencing earlier Facebook posts made by the historian.
He noted that Dr Thum and other activists met Malaysian Prime Minister Mahathir Mohamad last Thursday and invited him to "interfere in Singapore's internal politics".
After the meeting on Aug 30, Dr Thum wrote that he had urged Tun Dr Mahathir to "take leadership in South-east Asia for the promotion of democracy, human rights, freedom of expression and freedom of information".
A day later, the historian put up another Facebook post wishing Singaporeans a "happy unofficial independence day".
The group came under fire from some netizens after Marine Parade GRC MP Seah Kian Peng wrote in a Facebook post last Saturday that Dr Thum had invited Dr Mahathir to bring democracy to Singapore, and that "it appears quite clear to me that PJ Thum does not wish Singapore well".
On Sunday, Home Affairs and Law Minister K. Shanmugam referenced Dr Thum's Aug 30 post on Dr Mahathir and said "it is quite clear what that means".
The minister added that it is an absolute no-no to invite a foreign politician to intervene in Singapore's domestic politics.
Defending himself on Monday, Dr Thum said any notion that he is a traitor to Singapore is "ridiculous and unfounded".
He added: "I love my country and my people."
Mr Kausikan asked a day later: "But which is his country and who are his people? Not a straightforward issue when dealing with a slippery character."
The former ambassador-at-large pointed to several of Dr Thum's Facebook posts from 2013 to last year, which were shared on social media and in WhatsApp chat groups.
On Aug 31, 2013, Dr Thum wished Singapore a happy 50th illegal independence day.
Thum's earlier Facebook posts
Aug 31, 2013: 50 years ago today: Lee Kuan Yew, frustrated with the delay in the creation of Malaysia, unilaterally announced the independence of Singapore. In a rare show of unity, the international community rolled their eyes and ignored him. Happy 50th illegal independence day, Singapore!
July 22, 2014: Happy 107th birthday to Zubir Said, composer of Majulah Singapura, whose music reflects the soul of Malaya.
Aug 31, 2014: Selamat Hari Merdeka to the people of the former Federation of Malaya! Also, happy 51st illegal independence day to Singapore. By coincidence, Aug 31 is also the birthday of two of the three worst Roman emperors: Caligula (12 CE) and Commodus (161 CE).
Aug 9, 2016: Happy 51st National Day to all Singaporeans. On this day in 1965, we separated from Malaysia. It is my fervent wish that we will overcome narrow politics and one day return to our rightful place alongside our brothers and sisters in Malaya.
Aug 31, 2016: Selamat Hari Merdeka to all friends from the former Federation of Malaya! (And happy unofficial independence day to the people of Singapore, Sarawak, and North Borneo!)
Sept 16, 2017: Happy independence day to all Singaporeans! 54 years ago we achieved freedom from the British empire by joining Sarawak, North Borneo, and the Federation of Malaya to form the in-dependent and sovereign state of Malaysia. Selamat Hari Malaysia to all the peoples (past and present) of Malaysia!
Aug 31, 2018: Selamat Hari Merdeka to the people of the former Federation of Malaya! (And happy unofficial independence day to the people of Singapore!)
Mr Kausikan, citing the historian's statement, said that "even for him, (it is a) remarkable distortion of history", and noted that Dr Thum used the same phrase again in a 2014 post.
He added that Dr Thum had consistently referred to "Malaya" in his posts - "a loaded reference which, as a historian, he should recognise, and cannot be accidental".
Mr Kausikan also noted that on Aug 9, 2016, Dr Thum wrote that "it is my fervent wish that we will overcome narrow politics and one day return to our rightful place alongside our brothers and sisters in Malaya".
Malaya, comprising the states in the peninsula, became independent from British rule on Aug 31, 1957. It was only in 1963 that Sabah, Sarawak and Singapore joined the Federation of Malaya, which then became known as the Federation of Malaysia.
The inauguration of Malaysia was initially slated for Aug 31, 1963. But this was postponed by about two weeks to Sept 16 to give the United Nations more time to determine if the people of Sabah and Sarawak were agreeable to joining Malaysia.
Mr Kausikan added that Dr Thum is "still twisting and turning, trying to slip out of what he has said and done".
In comparison, graphic novelist Sonny Liew had publicly admitted he should have used better judgment over the meeting with Dr Mahathir, Mr Kausikan noted. "That is at least half-way honest."
Dr Thum could not be reached for comment yesterday.
Mr Liew, in response to questions from online news site Today, had written in a Facebook post on Monday that he and the rest who met Dr Mahathir were "much too wide-eyed and naive" in doing so.
He said the meeting "was a perfectly innocent if rare opportunity in our own minds", and added that in retrospect, they should have asked a lot more questions and been a lot more prepared for potential pitfalls.