Although the post is largely ceremonial, the president has make-or-break power over the nation's fragile ruling alliance and the power to dissolve parliament in the event of a crisis.
Since inconclusive elections in October, Portugal's minority Socialist government has been relying on a delicate coalition with the extreme-left to run the country of 10.4 million people.
The overwhelming favourite for the next head of state is a TV pundit, Marcelo Rebelo de Sousa.
The 67-year-old law professor has been involved in Portuguese politics and media since his youth, co-founding a weekly newspaper in his 20s and helping to establish the centre-right Social Democratic Party.
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Starting in the early 2000s he made his debut as a political analyst on TV, delivering clever commentary to a viewership that quickly grew.
"People love Marcelo because he is entertaining," said Rebelo de Sousa biographer Vitor Matos.
His popularity is widely expected to help him break the 50-per cent mark for an outright win in today's voting. If none of the 10 candidates breaches this threshold, a run-off will be held on February 14.
The government of Socialist Prime Minister Antonio Costa has promised to implement a moderate programme that upholds European Union budget commitments.
But it is forced to count on the support in parliament of a bloc of communists and greens that has not renounced its critical stance towards European budgetary rules and Portugal's membership of NATO.
Rebelo de Sousa is a long-time conservative who has the backing of right wing parties but who claims total independence.