Argentina

Presidential elections are held every 4 years at the same time as the legislative election, in a modified two-round electoral system, where the first-place candidate in the first round only needs to win 45% of the vote (to avoid a runoff), provided that they have a lead of at least 10% over the second-place candidate. An absolute majority (more than half of the votes, 50%+) is therefore not required for a first-round victory. In the second round, one candidate wins by a simple majority (excluding ties).

House of Representatives

In the case of the Chamber of Deputies (Cámara de Diputados de la Nación), closed-list, regionally proportional representation is used, more precisely, the D’Hondt method is used to distribute the seats between the parties. The 3% legal threshold is not valid nationwide, but per constituency, so it is not significant in most districts (most states elect 3-5 representatives, which inherently limits the proportionality of the results), but it may have a role in the province of Buenos Aires — where 35 representatives are elected at a time (Buenos Aires, as a city, is however a separate constituency, where only 12-13 representatives are elected, making it the second most populous district). However, considering the number of parties, despite the regional system, the D’Hondt method and the low district magnitude, the overall result can be quite proportional. This is particularly interesting considering that the apportionment of seats between the provinces has not been updated for 42 years (i.e. since the return to democracy), and to this day they are elected based on the data from the 1980 census (but not even proportionally to it). 

Senate

In the Senate (Senado de la Nación), the 3 seats per district are allocated by limited party-list block voting ('trinomial' system?), which can also be expressed as parties receiving fixed quotas of seats based on the results in the given province. This is also a closed list, meaning that you can only vote for parties, not for candidates within the lists (although this is what the primary election was used for until now, see the last paragraph). The party with the plurality always receives 2 seats, and the second-place finisher always receives one. In practice, the beneficiary of this (as opposed to a proportional formula) is mostly the first-place finisher, who is likely to gain an additional seat that would have gone to the third party in a proportional system. It seems that the Senate election (probably for this very reason) tends to be a fairly two-party race, but the same two parties do not necessarily compete everywhere. 

This is the description of the electoral system of Argentina as of 06.06.2026 on Electoral Knowledgebase. Sources and further information on this topic:

• Argentina | Election Passport 

Feedback, criticism is welcome and appreciated.