Chu Hao, a publisher and former deputy science minister, was accused by the party’s inspection committee of “grave errors” and of causing “negative impact on social thought”.
Mr Hao renounced his party membership in response, describing the allegations against him as groundless and malicious and intended to intimidate independent thinkers.
He faces unspecified punitive action for publishing books that backed more liberal and pluralistic politics.
A number of Mr Hao’s supporters took to social media to say they were also leaving the party because of its increasing intolerance of criticism.
The veteran writer, war veteran, and party member, Nguyen Ngoc, said that the party had become an arbitrary organisation that betrayed the people.
He said it wanted to keep the people ignorant for the benefit of self-serving individuals.
Others said that the party had turned against its original objectives, and was now cracking down on those who demanded human rights or protested against foreign influence.
The turning point for many critics came with the party’s repression of those protesting against China’s encroachments in the South China Sea, and its determination to crush an environmental movement that followed the Formosa disaster.
The decision to discipline Chu Hao reflects an attempt by the party leadership to reinforce Marxist ideology and its own authority.
The furore that followed shows that support remains in influential circles for a more tolerant and pluralistic system, even if the conservatives have the upper hand at the moment.