Nim had a reputation as one of the most independent-minded and outspoken members of the Khmer Rouge, and was eventually arrested, tortured and executed at Tuol Sleng security prison in 1977 during a party purge.

As his confession stated, he was a part of the party's radical People's Movement wing, the Pracheachollana, led by Um Sim and associated with the republican nationalist Son Ngoc Thanh.

He continued his work for the Democratic Party up to the 1955 elections, which handed power to Prince Norodom Sihanouk's Sangkum movement amidst an atmosphere of extreme political intimidation and possible vote-rigging.

Sihanouk, having effectively destroyed the ability of the Democratic Party and the socialist Pracheachon opposition to function, now made an attempt to co-opt young leftists into the Sangkum movement; amongst the prospective candidates Hou Yuon, Chau Sau, Uch Ven and Hu Nim all won seats.

By the early 1960s, Nim had joined the staff of the Sihanoukist daily newspaper Neak Cheat Niyum, and after a trip to Beijing was asked to form the Khmer-Chinese Friendship Association; he also travelled to Pyongyang and to Hanoi, where he met Ho Chi Minh.

The 1966 elections resulted in domination of the Sangkum by its rightist elements, though thanks to his local popularity, Hu Nim (along with Hou Yuon) was able to retain his seat, despite Sihanouk actively campaigning against him.

The situation was inflamed in March–April 1967 by a revolt in the far north-east of the country, the Samlaut Uprising, that was blamed by Sihanouk on left-wing agitation, and specifically, though most likely incorrectly, on the activities of the remaining openly leftist politicians: Hou Yuon, Khieu Samphan, Chau Seng and Hu Nim.

Hu Nim was later to write that he initially joined them, but returned to the capital after a few days, having been persuaded by senior cadre Vorn Vet that it might be profitable to continue engagement with Sihanouk and persist in anti-government agitation.

Nim was to continue in his post as Minister of Information after the 1975 Communist victory in the Cambodian Civil War, and the establishment of Democratic Kampuchea after the remaining Sihanoukists were purged from the administration.

The historiography of the Vietnamese-backed People's Republic of Kampuchea regime, which ousted the Khmer Rouge in 1979, was to emphasise Nim's role as a 'moderate' socialist, with the result that his story is still prominently featured in the Tuol Sleng museum.