First Republic of Vietnam (1955-1963)

After Ngô Đình Diệm established the Republic of Vietnam and became President, Sửu repeatedly sent letters demanding and advising President Diệm to make reforms and changes of policy within the government. As a result, he was labeled by Diệm's government to be a "political opposition."

In February 1959, he was elected as a congressman representing the Saigon constituency. He then joined the Great National Solidarity Front with Nguyễn Tường Tam to oppose the government. On April 26, 1960, he and 17 dignitaries signed the proclamation, later known as the "Caravelle Manifesto," criticizing the government's mistakes and demanding that the President make reforms in policies. This made him become a thorn in the eyes of the government. On the occasion of the failed coup on 11 November 1960, he was accused of supporting the coup by the government and imprisoned. On the night of 11 July 1963, he was sentenced to 8 years in prison by a special military court in Saigon along with Phan Quang Đán, Vũ Hồng Khanh, and Bùi Lương. When defending himself in court, he said: "If I am guilty, then I only have one crime, which is to expel the French from Saigon, sin for the sake of the Nation!."^[7]

On 31 July 1963, he was exiled to Côn Đảo Prison to serve his sentence. However, only 3 months later, another coup broke out, overthrowing and assassinating President Diệm, he was released from prison and returned to Saigon.

Chief of State of the Republic of Vietnam (1964-1965)

After the "Three Heads" crisis, on 8 September 1964, he was invited to the National Synod by the Provisional Leadership Committee (the Three Heads). On 27 September, the Synod elected Phan Khắc Sửu as President. He presided over the Synod that drafted the 20 October 1964 Covenant to replace the 4 November 1963 Provisional Charter, which placed power in the hands of the military to relinquish power and national sovereignty to elected representatives. On 24 October, he was nominated by the Synod for the position of Chief of State.^[8]

After taking office as Chief of State, on 4 November 1964, he appointed Trần Văn Hương as prime minister, he was the second civilian prime minister since the First Republic of President Ngô Đình Diệm was overthrown by the rebels. However, Trần Văn Hương's government was quickly paralyzed by the opposition of many circles as well as the lack of cooperation of the Military Council.^[9][10]

The crisis lasted for 2 months, on 18 December 1964, General Nguyễn Khánh led the young generals to establish the Military Council and two days later ordered the dissolution of the Synod. However, he was still retained as Head of State. On 27 January 1965, Prime Minister Hương was also forced to resign, handing over the Prime Minister's power to Deputy Prime Minister Nguyễn Xuân Oánh.

On 16 February 1965, General Nguyễn Khánh, in the name of Chairman of the Military Council, signed a decision to appoint Mr. Phan Khắc Sửu as the Chief of State and to appoint Dr. Phan Huy Quát as the Prime Minister to establish a new government. Less than 10 days later, on 25 February 1965, General Nguyễn Khánh was deposed by a group of young generals and had to accept the position of ambassador-at-large abroad. Less than four months later, on 5 June 1965, Prime Minister Phan Huy Quát's civilian government was dissolved by the Military Council. The young generals formed a National Leadership Council and appointed Lieutenant General Nguyễn Văn Thiệu as Chief of State. On 12 June 1965, Sửu and Prime Minister Phan Huy Quát officially left their positions of Chief of State and Prime Minister.^[11]

Famous for his integrity, as Chief of State, he only eats meals provided by the government and his salary is transferred to the social welfare fund for the people. In particular Sửu's wife, Mrs. Sửu, who is also a Cao Đài believer with the name Huỳnh Điệp, refusing to sit in the position of Madame, equivalent to First Lady. Mrs. Sửu rather sell clothes at Vườn Chuối Market in District 3 to pay for her life and take care of her children and grandchildren.

Second Republic of Vietnam (1967-1975)

In 1966, he returned to political activities, was once again elected a member of the National Assembly and was elected Chairman of the National Assembly. In 1967, Sửu became a candidate for president in the 1967 South Vietnamese presidential election, he finished third with 513,374 votes (10.8%).^[12] The joint venture of two generals Nguyễn Văn Thiệu - Nguyễn Cao Kỳ, prevailed winning 1,649,561 of the votes (34.8%).^[13] The National Assembly of the Republic of Vietnam met and voted, with 58 votes in favor and 42 votes against certifying the election. After this incident, Sửu resigned in protest of the military junta gaining the presidency that had a democratic name but could not reverse the situation. Fed up, he withdrew from politics.

In 1968, he together with a number of dignitaries such as Nguyễn Thành Vinh and Trần Sinh Cát Bình founded the Vietnamese New People Movement.^[14]