The Canadian political system, as well as that of Quebec, is a direct descendant of the British system, as clearly highlighted by the introduction of the Constitution Act of 1867.
Under this government structure, the role of prime minister is given, not to the leader of the party with the most MPs, but to the MP who is able to command the support of a majority in the House of Commons. Since this MP is usually the leader of the party that has elected the most MPs, we have the impression that this is the rule. However, we have several examples that differ from this model.
The first of these examples is the crisis leading to the Balfour Declaration and the Statute of Westminster of 1931. While the 1925 election had given 101 MPs to Mackenzie King’s Liberals and 116 to Arthur Meighen’s Conservatives, it was King who formed the government, because he was able to obtain a majority with the support of Robert Forke’s Progressives.
We have also seen prime ministers resign and be replaced by the newly elected leader of their party. John Turner replaced Pierre Elliott Trudeau, Kim Campbell succeeded Brian Mulroney, and Paul Martin took over from Jean Chrétien after the latter resigned.
British Comparable
The same kind of scenario happened in the United Kingdom when Boris Johnson left office. He was replaced by Liz Truss, the new leader elected by the members of the British Conservative Party.
A similar situation, but with one important difference, led to the arrival of Rishi Sunak as Prime Minister. After Liz Truss resigned, he was the only candidate from his party to have obtained the support of the 100 MPs needed to run for the Conservative Party leadership. He was never elected by the party’s members.
All of these situations follow the resignation of the leader. However, there is at least one case where the British Prime Minister was forced to resign due to the dissatisfaction of his MPs and the leadership of an opponent in his own party.
Michael Heseltine challenged his leader and Prime Minister, Margaret Thatcher. While he failed to take over the British Conservative Party, he demonstrated that Thatcher’s leadership was undermined. Had he succeeded, he would have become the leader and thus secured the position of Prime Minister, since he would have been the one leading the majority in the House of Commons.
This situation happened in the United Kingdom, but the formula also applies to Canada. So, if Justin Trudeau is still prime minister, it is not because he does not want to leave, but because there is no one in his party to challenge his leadership.