Eduardo Frei Montalva and his Christian Democratic Party would later unite with Allende's opponents to form a congressional majority in an attempt to declare his presidency illegal in August 1973, catalyzing the Chilean coup of 1973 a few weeks later.
The election was held using the absolute majority system, under which a candidate had to receive over 50% of the popular vote to be elected. If no candidate received over 50% of the vote, both house of the National Congress would come together to vote on the two candidates that received the most votes.
The CIA did not provide direct assistance to any candidate, as they had during the 1964 elections, but rather focused on anti-Allende propaganda, ultimately spending $425,000. The money was used in a "scare campaign" of posters and pamphlets linking an Allende victory with the violence and repression associated with the Soviet Union. Editorials and news stories reinforcing this message were also written with CIA guidance, especially in the newspaper El Mercurio, and disseminated throughout the national media. The goal was to contribute to and exploit the political polarization and financial panic of the period. Besides propaganda, the CIA also funded an attempt to splinter the Radical Party away from the Popular Unity coalition. This CIA campaign was very inefficient. CIA director Richard Helms complained that he was ordered by the White House to "beat somebody with nothing".
KGB money was more precisely targeted. Allende made a personal request for Soviet money through his personal contact, KGB officer Svyatoslav Kuznetsov, who urgently came to Chile from Mexico City to help Allende. The original allocation of money for these elections through the KGB was $400,000, and an additional personal subsidy of $50,000 directly to Allende. It is believed that help from KGB was a decisive factor, because Allende won by a narrow margin of 39,000 votes of a total of the 3 million cast. After the elections, the KGB director Yuri Andropov obtained permission for additional money and other resources from the Central Committee of the CPSU to ensure Allende victory in Congress. In his request on 24 October, he stated that KGB "will carry out measures designed to promote the consolidation of Allende's victory and his election to the post of President of the country".
US president Richard Nixon was enraged by the victory of Allende and by the failure of CIA covert actions.
As none of the candidates received an absolute majority of votes, the National Congress had to decide between the two candidates who had received the most votes, Allende and Alessandri. The precedent set on the three previous occasions this situation had arisen since 1932 was for Congress simply to choose the candidate with the highest number of votes; indeed, former President Alessandri had been elected in 1958 with 31.6% of the popular vote, defeating Allende.
In this case, however, there was an active campaign against Allende's confirmation by Congress, including an intensification of the CIA propaganda campaign to create concerns about Chile's future. During this period the CIA generated over 726 articles, broadcasts and similar items. The CIA also encouraged international economic pressure against Chile during this period. The United States also began to lay the groundwork for a military coup in this stage, authorizing the Ambassador to Chile to encourage this outcome with his contacts in the Chilean military.
Two days before the confirmation, Army Commander-in-Chief General René Schneider, was shot resisting a kidnap attempt by a group led by General Roberto Viaux. Hospitalized, he died of his wounds three days later. Viaux's kidnapping plan had been supported by the CIA, although it seems that then-US National Security AdvisorHenry Kissinger had ordered the plans turned off. Schneider was a known defender of the "constitutionalist" doctrine that the army's role is exclusively professional, its mission being to protect the country's sovereignty and not to interfere in politics, and had expressed his deep opposition to organizing a coup d'état in case Salvador Allende was finally chosen by the National Congress as President. Schneider's death was viewed negatively by the public, and helped citizens and military support Allende, whom the National Congress finally chose on 24 October. On 26 October, President Eduardo Frei named General Carlos Prats as commander in chief of the army in replacement of René Schneider.
Allende's presidency was eventually ratified, after he agreed to sign a "Statute of Constitutional Guarantees", promising not to undermine the Chilean Constitution.