The Mayor of Lisbon has been elected as the Socialist party’s candidate for prime minister at the next election.
Antonio Costo beat the incumbent Antonio Jose Seguro, picking up 70 percent of the party vote on Sunday.
The former justice and interior minister needs to maintain the party’s lead in the polls ahead of next October’s vote.
The Socialists will still fall short of a parliamentary majority even if their current lead is maintained.
Under a Social Democrat-led coalition, Portugal exited its EU bailout programme in May with the economy slowly recovering following the financial crisis.
Costo is likely to take the role of party leader following the resignation of Seguro on Sunday.