Harold Holt disappeared while swimming at Portsea, Victoria, on 17 December 1967, and was officially presumed dead two days later.
The Governor-General, Lord Casey, sent for McEwen and commissioned him as interim Prime Minister, on the understanding that his commission would continue only so long as it took for the Liberals to elect a new leader.
McEwen contended that if Casey commissioned a Liberal as interim Prime Minister, it would give that person an undue advantage in the upcoming ballot for a full-time leader.

McEwen retained all of Holt's ministers, and had them sworn in as the McEwen Ministry.
Approaching 68, McEwen was the oldest person ever to be appointed Prime Minister of Australia, although not the oldest to serve; Menzies left office one month and six days after his 71st birthday.
McEwen had been encouraged to remain Prime Minister on a more permanent basis but to do so would have required him to defect to the Liberals, an option he had never contemplated.

It had long been presumed that McMahon, who was both Treasurer and deputy Liberal leader, would succeed Holt as Liberal leader and hence Prime Minister.
However, McEwen sparked a leadership crisis when he announced that he and his Country Party colleagues would not serve under McMahon.
 McEwen is reported to have despised McMahon personally.
More importantly, McEwen was bitterly opposed to McMahon on political grounds, because McMahon was allied with free trade advocates in the conservative parties and favoured sweeping tariff reforms, a position that was vehemently opposed by McEwen, his Country Party colleagues and their rural constituents.

Another key factor in McEwen's antipathy towards McMahon was hinted at soon after the crisis by the veteran political journalist Alan Reid.
According to Reid, McEwen was aware that McMahon was habitually breaching Cabinet confidentiality and regularly leaking information to favoured journalists and lobbyists, including Maxwell Newton, who had been hired as a "consultant" by Japanese trade interests.

Even in the wake of their landslide victory in 1966, the Liberals were still four seats short of an outright majority.
With only the Country Party as a realistic coalition partner, McEwen's opposition forced McMahon to withdraw from the leadership ballot.
This opened the door for the successful campaign to promote the Minister for Education and Science, Senator John Gorton, to the Prime Ministership with the support of a group led by Defence Minister Malcolm Fraser.
Gorton was elected as leader of the Liberal Party on 9 January 1968, and succeeded McEwen as Prime Minister the following day.
It was the second time the Country Party had effectively vetoed its senior partner's choice for the leadership; in 1923 Earle Page had demanded that the Nationalist Party, one of the forerunners of the Liberals, remove Billy Hughes as leader before he would even consider coalition talks.
Harold Holt disappeared while swimming at Portsea, Victoria, on 17 December 1967, and was officially presumed dead two days later. The Governor-General, Lord Casey, sent for McEwen and commissioned him as interim Prime Minister, on the understanding that his commission would continue only so long as it took for the Liberals to elect a new leader. McEwen contended that if Casey commissioned a Liberal as interim Prime Minister, it would give that person an undue advantage in the upcoming ballot for a full-time leader.
McEwen retained all of Holt's ministers, and had them sworn in as the McEwen Ministry. Approaching 68, McEwen was the oldest person ever to be appointed Prime Minister of Australia, although not the oldest to serve; Menzies left office one month and six days after his 71st birthday. McEwen had been encouraged to remain Prime Minister on a more permanent basis but to do so would have required him to defect to the Liberals, an option he had never contemplated.
It had long been presumed that McMahon, who was both Treasurer and deputy Liberal leader, would succeed Holt as Liberal leader and hence Prime Minister. However, McEwen sparked a leadership crisis when he announced that he and his Country Party colleagues would not serve under McMahon.  McEwen is reported to have despised McMahon personally. More importantly, McEwen was bitterly opposed to McMahon on political grounds, because McMahon was allied with free trade advocates in the conservative parties and favoured sweeping tariff reforms, a position that was vehemently opposed by McEwen, his Country Party colleagues and their rural constituents.
Another key factor in McEwen's antipathy towards McMahon was hinted at soon after the crisis by the veteran political journalist Alan Reid. According to Reid, McEwen was aware that McMahon was habitually breaching Cabinet confidentiality and regularly leaking information to favoured journalists and lobbyists, including Maxwell Newton, who had been hired as a "consultant" by Japanese trade interests. 
Even in the wake of their landslide victory in 1966, the Liberals were still four seats short of an outright majority. With only the Country Party as a realistic coalition partner, McEwen's opposition forced McMahon to withdraw from the leadership ballot. This opened the door for the successful campaign to promote the Minister for Education and Science, Senator John Gorton, to the Prime Ministership with the support of a group led by Defence Minister Malcolm Fraser. Gorton was elected as leader of the Liberal Party on 9 January 1968, and succeeded McEwen as Prime Minister the following day. It was the second time the Country Party had effectively vetoed its senior partner's choice for the leadership; in 1923 Earle Page had demanded that the Nationalist Party, one of the forerunners of the Liberals, remove Billy Hughes as leader before he would even consider coalition talks.
John McEwen