Trained as a doctor, Đán first entered politics in 1945 when the Japanese occupation of Vietnam ended and several local groups challenged French attempts to re-establish colonial power.

Despite ransackings by a mob of regime supporters, the newspaper's closure by a government court, and his blacklisting from university employment, Đán continued his opposition activities and was elected to the National Assembly in 1959, but was prevented from taking his seat.

[4] According to his account, he twice turned down Vietminh offers of a cabinet position in 1946 to follow Emperor Bảo Đại to China and Hong Kong.

He resigned after several months, citing the French reluctance to grant the government any powers to facilitate Vietnamese autonomy, noting that they wanted to 'reestablish the old colonial regime'.

[5] In 1949, Đán formed his own group, the Republican Party (Cong Hoa Dang) and went abroad to study for his PhD at the Harvard School of Public Health while continuing his political activities.

Đán said that it was due to Diem being appointed by Bảo Đại, but the government maintained that it was because he was holding out for a more important ministry,[4] having allegedly rejected an offer to become the Minister for Social Welfare.

[2] For his part, Đán later claimed that he rebuffed Diem because he 'never intended to cooperate with Diệm', who he asserted could not administer a government that could modernise Vietnam in a democratic manner, but instead was set on feudal and nepotistic rule.

[2] Đán claimed that upon his return to Vietnam in September 1955, Diem's officials sought him out at the airport to arrange a meeting at the Norodom Palace.

[2] Đán claimed that he reprimanded Diem for running a nepotistic regime and relying on the counsel of his younger brother Ngo Dinh Nhu, and stated his intention to contribute to South Vietnamese politics by organising 'a constructive, legal opposition'.

Đán was briefly arrested on the eve of the 1956 elections, and accused by government controlled media of involvement in communist and colonialist activities.

[7] Đán openly criticized the main platform of American economic development aid to South Vietnam, the Commercial Import Program.

"[11] On 30 August 1959, Đán ran for the National Assembly in a constituency in Saigon and was elected by a 6–1 ratio over Diem's government candidate.

When the Assembly was inaugurated, Đán was confronted by police and put under arrest as he attempted to leave his medical clinic to attend the session.

[12] Đán was charged with electoral fraud, on the grounds that he supposedly offered free medical care to induce voters to support him.

[14] Đán spoke on Radio Vietnam and staged a media conference during which a rebel paratrooper pulled a portrait of the president from the wall, ripped it and stamped on it.

Diem then crushed the rebels and Đán was arrested, tortured and sentenced to eight years of hard labour in the penal colony on Poulo Condore where the French had once imprisoned Vietnamese nationalists.