Two weeks before the 1935 provincial election, he engineered a coalition with the Action libérale nationale (ALN), a party of dissident reform Liberals and nationalists who had quit the governing Parti libéral du Québec. While he lost that election, Duplessis was soon able to exploit a patronage scandal involving the family of Premier Louis-Alexandre Taschereau to force Taschereau's resignation. The A.L.N. and Conservatives had by now formally merged into a single party, the Union Nationale.
Duplessis and the UN won the August 1936 election in a landslide, putting an end to thirty-nine consecutive years of Liberal rule. Duplessis's first government was defeated in the 1939 election, a snap election called by the premier in hopes of exploiting the issue of Canadian participation in World War II.
Duplessis returned as premier in the 1944 election, and held power without serious opposition for the next fifteen years, until his death. He became known simply as le Chef (the chief, the boss). He was elected to five terms of office in all, the last four of them consecutive. After him, no political party in Quebec elections at the provincial level has managed to win more than two terms of office in a row.
Duplessis favoured rural areas over city development and introduced various agricultural credits during his first term. He also was noted for meagre investment in social services and effective electoral campaigning.
During his terms, while saying that he was not against unions, he made several laws that drew heavy criticism from especially the international union groups such the Trades and Labour Congress of Canada (TLC). His most notable antiunion initiative was the Padlock Law, which initially was a law that would eliminate Communist propaganda.
In 1949, Duplessis also tried to introduce a copycat law of the U.S Taft-Hartley Act, created in 1947, which would have eliminated certain rights for union groups that were acquired by the Labour Relations Law of 1944, the equivalent of the American Wagner Act of 1935. It was withdrawn due to the fierce opposition by union groups later reintroduced a nearly similar law in 1954, known as Bill 19. The law would force union groups to ban any members that would support Communism and any group would lose its trade-union accreditation if any there is a single member that had ties with Communist groups or supported the ideology. The party lost even the support of the Catholic union group and forced it to review its structure which would lead to the creation of the Confédération des syndicats nationaux (CSN).
During Duplessis' mandates, several significant labour strikes occurred such as the Dominion Textile in Valleyfield in 1946, the asbestos in the Beauce region in 1949 and the Murdochville copper mine strike in 1957. In those conflicts, Duplessis responded rapidly with force, using the provincial police to disperse picket lines and restore order. Several arrests were made in these conflicts. However, the latter led to a major victory to union groups which acquired several rights.
The UN often had the active support of the Roman Catholic Church in its political campaigns. Referring to the two parties' campaign colours, a slogan commonly heard from the pulpit was Le ciel est bleu; l'enfer est rouge: The sky/heaven is blue (UN); Hell is red (Liberal). Only during the labour strikes in the 1950s did the Church break with the UN by supporting the unions. Duplessis also actively opposed Jehovah's Witnesses and once used his influence to revoke a liquor license from one of their organizer's businesses. This decision was overturned by the Supreme Court of Canada.
On January 21, 1948, he made one of his most enduring contributions to Quebec with the adoption of an official Flag of Quebec, the fleurdelysé, which replaced the Union Flag at the top of the Quebec Parliament Building.
Days before the election of 1960, the chief organizer for the Union Nationale was arrested for fraud when 4,000 fake voters' slips were found inside a train station locker. UN leader Daniel Johnson denied the charges saying it was a Liberal frame-up. Jean Lesage responded by saying corruption, blackmail and political immorality were trademarks of the Union Nationale from the Maurice Duplessis days.
He died in office in Schefferville, Quebec, on September 7, 1959. Afterwards, Quebec society was caught in a very swift socio-cultural change away from his conservative, Church-oriented policies toward a highly secular, socially liberal welfare state. This was called the Quiet Revolution (Révolution tranquille) and also went by simultaneously with the radical church reforms during and after the historicizing Second Vatican Council (1962-1965), which highly influenced the previously very Catholic region of Quebec. Many of these major changes occurred when the Liberals regained power in 1960 under Jean Lesage.
The Quebec born newspaper baron Conrad Black wrote a biography, Duplessis (ISBN 0-7710-1530-5), now out of print.
The period of Duplessis's reign is often referred to in Quebec as "The Great Darkness" (La Grande Noirceur) due to his very conservative, rural, corrupt, and Church-oriented policies and for the notorious "Duplessis Orphans". The nickname also referred to the opposition Liberals' unsuccessful attempts to challenge Duplessis' power during that time. He once proclaimed that a much-needed bridge at Trois-Rivières would not be built should a Liberal MNA be elected and kept his word while the opposition held the seat. In another rural district, the residents decided in 1956 to vote for the UN as that was the only way to get new roads constructed.
Most of his surviving relatives have not handed down the "Duplessis" name to their children, although one of his nieces, Berthe Brunet-Dufresne, has taken it upon herself to rehabilitate her uncle.
*The Union Nationale was founded as an alliance in 1935 with Duplessis as leader. In 1936 the UN formally became a unitary political party with the Quebec Conservative Party dissolving into it.