Coalition deals are expected if the vote is close
Portugal votes on Sunday for a new assembly after the president dissolved the last one in December, a move which triggered the government's resignation.
Polls show the opposition Socialists ahead of the outgoing Social Democrats, in a contest widely seen as about halting economic decline.
Q: Why was the election called?
Social Democrat Prime Minister Pedro Santana Lopes took office last July after his predecessor, Jose Manuel Barroso, left to head the European Commission.
The Lopes government was beset by flagging polls and allegations of ministerial incompetence, including blame for a chaotic start to the school year.
President Jorge Sampaio resisted calls for early elections. But in December, he said dissolving parliament was the only way out of a "crisis of instability".
Since then Mr Lopes has led a caretaker administration.
Social Democrats: 105 seats (40.1%)
Socialists: 96 seats (37.8%)
Popular Party: 14 seats (8.7%)
Communists/Greens: 12 seats (6.9%)
Left Bloc: 3 seats (2.7%)
Q: What happened last time?
After the 2002 election, the centre-right Social Democrats and rightist Popular Party formed a coalition government.
Their combined majority was 119 parliamentary seats out of 230
Q: Who are the main contenders?
The Social Democratic Party, led by 48-year-old Pedro Santana Lopes. The party is centre-right and akin to the UK's Conservative Party, the German CDU or Spanish Popular Party.
Mr Lopes has served as mayor of Lisbon and Figueira da Foz. He has been a secretary of state for culture and chairman of a Lisbon football club.
The Socialist Party, headed by 47-year-old Jose Socrates. The party is similar to the UK's Labour Party or German SPD.
Mr Socrates was environment minister in the government of Antonio Guterres.
Q: Who else is standing?
The Popular Party, led by the defence minister in the last government, 42-year-old Paulo Portas. A Christian Democratic party similar to the German CSU.
With 14 seats in parliament, it was the Social Democrats' coalition partner.
The Communist Party, led by its new secretary-general, 57-year-old Jeronimo de Sousa.
A former metalworker, he is known as a hardline Marxist-Leninist. He rejects forecasts of the party's demise.
The Left Bloc, led by 48-year-old Francisco Louca. Formed in 1999, it advocates leftist policies including legalisation of abortion and soft drugs.
Marktest poll (28 Jan)
Social Democrats: 27.7%
Correio da Manha (11 Feb)
Socialists: 44.7% Social Democrats: 27.4%
Catholic University (18 Feb)
Socialists: 46% Social Democrats: 31%
Q: What do opinion polls say?
Polls have given the Socialists 44-46% of the vote and the Social Democrats 27-31%.
Mr Lopes has denounced the findings as "mega-fraud" and threatened to sue pollsters if the outcome proves different.
Q: What might happen?
The Social Democrats and the Popular Party are standing separately but have vowed to form a new alliance to keep the Socialists out.
The Socialists have set their sights on a majority win. A coalition with the Communists is seen as unlikely, but one with the Left Bloc is not ruled out.
What are the issues?
Political stability: Financial analysts say this is needed to restore investor and consumer confidence.
Deficit: The 2005 budget sets a deficit target of 2.8% of GDP. Many doubt this is achievable unless a new government introduces drastic spending cuts. All parties have called for reform of the EU Stability Pact.
Growth: The Socialists have ruled out tax cuts and pledged to kick-start growth through investment in technology and training. The Social Democrats have vowed to bring productivity back to 75% of the EU average from 64%.
Europe: Both the main parties back the new European Union constitution and have promised a referendum. The Communists and the Left Bloc oppose the treaty.
Abortion: This is currently banned unless the mother's life is threatened. The Socialists want a referendum on easing the law. The Communists and the Left Bloc back this, while the Popular Party is firmly against. The Social Democrats are divided.
Q: What is parliament's role?
The Assembly of the Republic is unicameral and elected for four years.
The president can dissolves the Assembly and call new elections. The parliament speaker stands in for the president during his absence.
The prime minister names a Council of Ministers. A new government must present its legislative programme to parliament for approval.
Q: What is the system?
Out of 230 MPs, 226 are elected from party lists in 22 constituencies by a system of proportional representation.
The other four are elected by Portuguese citizens abroad.
There are some 8.7m eligible voters. If no single party gets a majority (at least 116 seats) the president will ask the largest party to form a coalition.
Q: How soon will results be known?
The law says exit polls can be announced only after voting ends in the Azores, one hour later than elsewhere. Final results - excluding votes abroad - are expected Sunday night.
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