Allende defeats Pinochet and purged rightwing militaries (replacing Armed Forces with militias is absurd, he was elected democratically as head of state of Chile and was a strong leftwing socialist, but not a revolutionary. Overall many officers were loyal to the Constitutional Order, as Carlos Prats and René Schneider).
Some Christian Democracy rightwing politicians are arrested because they plotted with Pinochet.
Allende revealed CIA connections wit plotters, creating a public scandal and international outrage.
In 1974 right-wingers tried to start an impeachment against Allende, as opposition controlled the National Congress following 1969 election (as they were planning that OTL).
But we have to consider some things:
- Allende was not a communist dictator, as many oppositors tried to paint him. A simple prove is that in 1969 and 1973 parliamentary elections opposition won and, due this, he made deals with it: his agrarian reform couldn't pass without Christian Democrats votes (and they voted for as, well, it was a good Christian thing) and CDP voted to make him President in 1969.
- Chilean people, although terror campaigns by communists and fascist terrorists, supported Allende presidency and policies. A simple prove is that during 1973 parliamentary election right opposition actually lost votes and seats in favor of Allende's Popular Front.
- Allende's opposition in Congress (Confederation of Democracy; CODE) was a coalition formed by a great mess of parties (rightwing National Party, center-right Radical Party, Christian Democratic Party, center-left Radical Liberal Party and populist National Democratic Party) with only target to take two-third majority to remove Allende and pass some authoritarian constitutional reforms, but its members were not all rightwing conservatives. There were moderates and also left-wingers who voted for Allende's reforms, so it's improbable that all opposition members are willing to impeach Allende.
- at the opposite Allende's Popular Unity (composed by Socialist Party, Communist Party, center-left Radical Party, leftwing Social Democratic Party, christian socialists Popular Unitary Action Movement, PUAM Worker-Peasant and Christian Left Party and populist Independent Popular Action) was a pretty compact coalition and this prevented major defections.
- some Allende's macroeconomic policies were bad for Chilean economy or didn't work, but nevertheless they were popular: nationalizing copper mines and approving the biggest plan against poverty in previous Chilean history were widely popular, especially among poors. Also political impasse contributed to crisis as Congress rightwing majority refused to collaborate with Allende (and precisely it lost votes).
- after CIA Scandal, we can expect a major popular backlash against America and pro-coup anti-Allende "CIA-puppets" politicians. Under a such popular pressure it's hard find many parliamentarians willing to depose Allende.
Overall, even if all opposition members voted for impeachment, CODE actually had not enough votes to reach two-third Senate majority to remove Allende.
So impeachment fails and badly backfires against proponents.
In 1976 there is a new presidential election: as the Chilean Constitution officially bar to run for two consecutive terms, Allende is excluded from the race. Unity Popular needs to choose a popular figure to win reelection and has two main candidates: Communist Senator, famous leftwing writer and Culture Nobel Prize Pablo Neruda (Communist candidate in 1970, before endorsing Allende) and Senator and Socialist Secretary Carlos Altamirano. Neruda had cancer before be assassinated by Pinochet and a Communist candidate would prove too controversial to win election, so it's more probable Altamirano. The CODE can choose between former Christian Democratic President Eduardo Frei and conservative National Party Senator and anti-socialist editor Sergio Onofre: as the later proved to be too unbalanced towards right (Christian Democratic Party is always the main party), Frei is chosen in a comeback typical of Chilean politics. As Frei supported Pinochet failed coup and Allende's impeachment attempt he can count on right-wingers votes, while his moderate policies allow him to have good relationship with centrists. However, as CDP and CODE shifts strongly towards right under Nationalists pressure, Christian Democrats leftwingers as Bernardo Leighton and Radomorico Tomic refuse to actively support Frei and probably start an alternative candidacy (Leighton is the favorite for the role) claiming to be moderate who can fix country's problems respcting the Constitution, promoting welfare and defending democracy.
Economic problems are a damage for Left reputation but they can say that rightwing Congress prevent action to solve them (and protect landlords,...) and CIA Scandal works in Peft favour.
At the end CODE split allow Altamirano to narrowly win popular vote but, as no one can win a 50% plus one of votes, the Congress has to choose between two most voted candidates and, thanks its rightwing majority, elects Frei to the Presidency. The Left is outraged as it was a long-term tradition for the Congress to simply ratifie the results, electing the most voted candidate (as for Allende in 1970). Thanks to this "electoral bargain", Unity Popular increases its seats in 1977 parliamentary elections but CODE retains a slim majority, due the promise to fix economy. Frei immediately forms a Goverment with militaries and rightwing politicians, with new Interior Minister Onofra launching a repression campaign against communists (who start some riots against "Frei fraud"). Frei restores good relationship with US and, in order to stabilize economy and get foreign loans, privatizes mining sector and starts an austerity plan, a move hugely unpopular. The international Recession of 1979 reinforces negative results. Also military humiliation during the short Beagle War against Argentina in 1978 is seen as a major blow for second Frei's Presidency. In 1981 Popular Unity wins parliamentary elections for the first time and forces Onofra to resigne. They try also to overrule austerity programs but the attempts are vetoed by Frei.
In 1982 Allende returns and win soundly against Onofra, who, as Interior Minister, assumesd the Presidency after Frei's death. His election is confirmed by the Congress and one of the first legislation is a bipartisan (it was long discussed before be adopted OTL after end of Pinochet's regime) constitutional reform toward a President elected directly by popular vote by runoff. Allende follows Altamirano's advice and adapted his socialist vision to Eurocommunism, finding friend both in Western Europe and in new freed Latin American countries. Reagan try to finance opposition against him but this is revealed during Irangate and boosts Allende popularity. During his second Presidency Christian Democratic Party returns to a more moderate approach, splitting from National Party. In Left camp there similar problems between socialists and communists. Neruda dies and receives a state funeral. In 1988 Altamirano defeats Patricio Alwyn mainly due Allende's popularity but during 1990s the South America Crisis that hit mainly Brazil, Venezuela, Argentina and Mexico hurts fragile Chilean populist economic policies, allowing Eduardo Frei Ruiz-Tangle to win in 1994 against Foreign Minister Ricardo Lagos. Then follows a normal elections history.
Presidents of Chile:
28 Salvador Allende Gossens 1970-1976 Socialist Party-Popular Unity
Defeated in Congressional Vote Jorge Alessandri (Conservative Independent)
29 Eduardo Frei Montalva 1970-1982 Christian Democratic Party-Confederation of Democracy (Died in Office)
Defeated in Congressional Vote Carlos Altamirano (Popular Unity)
30 Sergio Onofre Jarpa 1982 National Party-Confederation of Democracy
31 Salvador Allende Gossens 1982-1988 Socialist Party-Popular Unity
Defeated in Congressional Vote Sergio Onofre (National Party-Confederation of Democracy)
32 Carlos Altamirano Orrego 1988-1994 Socialist Party-Popular Unity
Defeated in Runoff Patricio Alwyn (Christian Democratic Party)
33 Eduardo Frei Ruiz-Tangle 1994-2000 Christian Democratic Party
Defeated in Runoff Ricardo Lagos (Socialist Party-Popular Unity)
34 Ricardo Lagos Escobar 2000-2006 Socialist Party-Popular Unity (Presidential term is reduced to four years)
Defeated in Runoff Andrés Zaldivar (Christian Democratic Party)
35 Sebastian Pinera Echenique 2006-2010 National Renewal
Defeated in Runoff Jorge Arrate (Socialist Party-Popular Unity)
36 Ricardo Lagos Escobar 2010-2014 Socialist Party-Popular Unity
Defeated in Runoff Eduardo Frei Ruiz-Tangle (Christian Democratic Party)
37 Sebastian Pinera Echenique 2014-2018 National Renewal
Defeated in Runoff Andrés Velasco (Independent supported by Socialist Party-Popular Unity)
38 Michelle Bachelet 2018-... Socialist Party-Popular Unity
Defeated in Runoff Evelyn Matthei (National Renewal)