The centre-left party of Prime Minister Antonio Costa is one of two main rivals that have dominated Portugal's political landscape since the end of Antonio Salazar's dictatorship in 1974. It has been in government the longest since then.
Costa, 60 and a former mayor of Lisbon, has led two consecutive minority governments since 2015, when the Socialists, with support from the hard left, unseated a centre-right coalition government that had presided over four years of tough austerity under an international bailout.
His pioneering pact with the Communists and Left Bloc for support in parliament ended in 2019. This ultimately led to the rejection of the 2022 budget bill in October, which triggered this snap election.
Under Costa, Portugal achieved solid economic growth and its first budget surplus in 2019 under democracy, winning praise from its European partners. The hard left argued he is too focused on spending controls.
The PS had 108 seats in the 230-seat parliament after winning 36 per cent of the vote in 2019. Opinion polls give it similar levels of support now.
Social Democratic Party
The centre-right party, which has been the Socialists' main rival for decades, on November 27 re-elected moderate economist and former mayor of Porto, Rui Rio, as its leader, ending a long period of internal uncertainty.
The PSD, which promises to cut corporate taxes to spur growth while keeping spending in check, has gained ground in opinion polls since then, and is running hot on Costa's heels.
Rio, 64, has suggested the PSD should allow the Socialists to govern for "at least two years" if the latter win the vote without a majority, so that Portugal can carry out reforms and make the most of a windfall of EU pandemic recovery aid. He hopes the Socialists would do the same if his party wins.
The bloc reached the peak of its popularity on a wave of anti-austerity protests, winning 19 seats in 2015. Aside from its many legislative proposals defending salaries, pensions and the welfare state, it has championed civil rights.
Catarina Martins, a 48-year-old actress-turned-politician, has struck a chord in Portugal's male-dominated politics by mixing an often tough message with a soft delivery.
With some voters blaming Left Bloc for the early election, opinion polls show it would lose some seats and possibly its title of the third-largest force in parliament.
Vying for that title is the populist, far-right Chega. Formed in 2019, it won one parliament seat the same year - the first for a far-right party since the end of the dictatorship, and could win over a dozen now.
It owes much of its growing popularity to its tough-talking leader, former sports commentator Andre Ventura, 39.
Often borrowing populist rhetoric from former US President Donald Trump's book and encouraged by the fast rise of similarly-minded anti-immigration, anti-feminist Vox in neighbouring Spain, political analysts see Chega as too toxic a potential partner for any other party in Portugal.
Led by former metalworker Jeronimo de Sousa, 74, who has honed his skills chastising capitalism for almost five decades in parliament, the party moderated its stance after the 2015 pact with Costa, ditching calls to leave the euro zone.
Polls show the party, which is allied with the Greens, is likely to lose some of the 10 seats it had in the 2019 legislature that it helped to bring down.
The conservative CDS-PP is PSD's traditional ally, but has been bleeding voter support to new right-wing rivals Chega and Liberal Initiative (IL) and risks losing all but one of its five seats. The IL, which had just one seat, could take five or more.
Pan (People, Animals, Nature)
The environmentalist animal rights party PAN has sided with the Socialists on various occasions and is seen as a potential kingmaker, along with the eco-Socialist party Livre, to the centre-left, if they get a slim majority altogether with the PS.