Citizens (C's) benefited from its anti-independence stance and climbed to second place ahead of a declining Socialists' Party of Catalonia (PSC), which scored a new historical low for the third election in a row.

The People's Party (PP) suffered from its national counterpart decline and scored its worst result since 1992, whereas the left-wing Popular Unity Candidacy (CUP) saw a strong performance which allowed it to hold the key to government formation with JxSí.

[c] Voting for the Parliament was on the basis of universal suffrage, which comprised all nationals over 18 years of age, registered in Catalonia and in full enjoyment of their political rights.

[3] The 135 members of the Parliament of Catalonia were elected using the D'Hondt method and a closed list proportional representation, with an electoral threshold of three percent of valid votes—which included blank ballots—being applied in each constituency.

[2] The president had the prerogative to dissolve the Parliament of Catalonia and call a snap election, provided that no motion of no confidence was in process and that dissolution did not occur before one year had elapsed since a previous one under this procedure.

Artur Mas explained in a public act on 25 November his plan to reach independence, proposing calling an extraordinary regional election—turned into an alternative vote on independence—at some point during 2015, on the condition that ERC agreed to join a common list with his party to stand together at the polls.

ERC leader Oriol Junqueras agreed with most of the plan but initially refused such a joint list, threatening to break its government pact with CiU in order to force an election in early 2015.

[9][10] After weeks of calibrated brinkmanship from both sides, with CDC pushing for a joint candidature to cover for its forecasted loss of support and ERC refusing to run with Artur Mas as presidential candidate, both parties finally reached an agreement, and on 14 January 2015, Mas announced that a snap regional election would be held on 27 September that same year, with the intention to turn in into the true plebiscite on independence.

[1] Aside from the pact to hold an extraordinary election, the agreement also included to complete state structures as a basic element to culminate the process of "national transition" as well as negotiation of budgets.

[13] The Spanish government, in response to the election announcement eight months ahead of the scheduled date, accused Mas of having "no interest in attending the Catalan people's problems, nor it has any capacity to solve them".

[19] On 17 June, after a meeting of the UDC leadership, it was announced that the party was withdrawing all three of its members from the Government of the Generalitat of Catalonia, although they agreed to maintain parliamentary stability until the end of the legislature.

On 3 August 2015, Catalan president Artur Mas signed the election decree and highlighted the extraordinariness of the proposal's background, which nonetheless did not mention the word plebiscite.

[35] Artur Mas was named as the agreed presidential candidate, even though, as a result of balance of power negotiations between ERC and CDC, he was placed 4th in the electoral ticket.

[46][47] The coalition was thus scheduled to comprise the ruling centre-right CDC; its supporting centre-left partner in parliament, ERC; DC and MES, pro-independence splits from UDC and PSC, respectively; and members from separatist sectors of the civil society.

[49] After the success of Ada Colau's Barcelona en Comú platform in the 2015 Barcelona municipal election, its member parties Podemos, Initiative for Catalonia Greens (ICV) and United and Alternative Left (EUiA) entered talks for coalescing into a similar, regional-wide coalition for the September election to run as an alternative to Mas' independence plan.

[50][51] By 15 July 2015, negotiations between the parties were already close to success, and it was agreed that they would stand together in the Catalunya Sí que es Pot electoral platform (English: Catalonia Yes We Can).

A last minute deal was struck between Junts pel Sí and Popular Unity Candidacy to ensure a separatist government, although without Mas as president.