In the 1963 general election, the Christian Democrats lost almost one million votes, gaining nearly 38%, while the Italian Communist Party (PCI) arrived second with 25%.
However the Italian Liberal Party (PLI) surged to 7%, their best results ever, receiving many votes from former Christian Democratic supporters, who were against Amintore Fanfani's centre-left policies.
With the decline of electoral support, on 22 June 1963, the majority of DC members decided to replace Fanfani with a provisional government led by Leone.

Leone formed a one-party cabinet, composed only by DC's members and externally supported by Italian Socialist Party (PSI), Italian Republican Party (PRI) and Italian Democratic Socialist Party (PSDI).
The cabinet was also known as "Bridge Government" (Governo ponte), as a transitional government, with the aim of starting a tighter cooperation with the PSI.

As Prime Minister, Leone had to face one of the most tragic events in Italian republican history, the Vajont Dam disaster.
On 9 October 1963, a landslide occurred on Monte Toc, in the province of Pordenone.
The landslide caused a megatsunami in the artificial lake in which 50 million cubic metres of water overtopped the dam in a wave of 250 metres (820 ft), leading to the complete destruction of several villages and towns, and 1,917 deaths.
In the previous months, the Adriatic Society of Electricity (SADE) and the Italian government, which both owned the dam, dismissed evidence and concealed reports describing the geological instability of Monte Toc on the southern side of the basin and other early warning signs reported prior to the disaster.

Immediately after the disaster, government and local authorities insisted on attributing the tragedy to an unexpected and unavoidable natural event.
However, numerous warnings, signs of danger, and negative appraisals had been disregarded in the previous months and the eventual attempt to safely control the landslide into the lake by lowering its level came when the landslide was almost imminent and was too late to prevent it.
The communist newspaper L'Unità was the first to denounce the actions of management and government.
Leone accused the PCI of political profiteering from the tragedy, promising to bring justice to the people killed in the disaster.
However, a few months after the end of his premiership, he became the head of SADE's team of lawyers, who significantly reduced the amount of compensation for the survivors and ruled out payment for at least 600 victims.

In December 1963, after only five months of government, when the congress of the PSI authorized a full engagement of the party into the government, Leone resigned and Aldo Moro, the secretary of the DC and leader of the more leftist wing of the party, became the new Prime Minister, ruling Italy for more than four years.
In the 1963 general election, the Christian Democrats lost almost one million votes, gaining nearly 38%, while the Italian Communist Party (PCI) arrived second with 25%. However the Italian Liberal Party (PLI) surged to 7%, their best results ever, receiving many votes from former Christian Democratic supporters, who were against Amintore Fanfani's centre-left policies. With the decline of electoral support, on 22 June 1963, the majority of DC members decided to replace Fanfani with a provisional government led by Leone.
Leone formed a one-party cabinet, composed only by DC's members and externally supported by Italian Socialist Party (PSI), Italian Republican Party (PRI) and Italian Democratic Socialist Party (PSDI). The cabinet was also known as "Bridge Government" (Governo ponte), as a transitional government, with the aim of starting a tighter cooperation with the PSI.
As Prime Minister, Leone had to face one of the most tragic events in Italian republican history, the Vajont Dam disaster. On 9 October 1963, a landslide occurred on Monte Toc, in the province of Pordenone. The landslide caused a megatsunami in the artificial lake in which 50 million cubic metres of water overtopped the dam in a wave of 250 metres (820 ft), leading to the complete destruction of several villages and towns, and 1,917 deaths. In the previous months, the Adriatic Society of Electricity (SADE) and the Italian government, which both owned the dam, dismissed evidence and concealed reports describing the geological instability of Monte Toc on the southern side of the basin and other early warning signs reported prior to the disaster. 
Immediately after the disaster, government and local authorities insisted on attributing the tragedy to an unexpected and unavoidable natural event. However, numerous warnings, signs of danger, and negative appraisals had been disregarded in the previous months and the eventual attempt to safely control the landslide into the lake by lowering its level came when the landslide was almost imminent and was too late to prevent it. The communist newspaper L'Unità was the first to denounce the actions of management and government. Leone accused the PCI of political profiteering from the tragedy, promising to bring justice to the people killed in the disaster. However, a few months after the end of his premiership, he became the head of SADE's team of lawyers, who significantly reduced the amount of compensation for the survivors and ruled out payment for at least 600 victims.
In December 1963, after only five months of government, when the congress of the PSI authorized a full engagement of the party into the government, Leone resigned and Aldo Moro, the secretary of the DC and leader of the more leftist wing of the party, became the new Prime Minister, ruling Italy for more than four years.
Giovanni Leone