Portugal has gone to the polls, in a vote that will either keep the centre-left in power or shift the country to the right.
Opinion polls suggest neither the Socialists nor the opposition centre-right is likely to win outright.
This could leave the far-right Chega party as kingmaker, although mainstream rivals have vowed to sideline it.
Mr
Costa was not named as a suspect, but investigators arrested his chief
of staff over alleged irregularities in state contracts.
The case has handed ammunition to André Ventura, who leads the Chega (Enough) party and has focused his campaign on corruption and immigration.
Despite
economic growth under the Socialists, Portuguese voters worry about
high inflation, deteriorating public services, and a housing crisis.
Mr
Ventura, a former centre-right councillor and one-time trainee priest,
made his name on national television as a football commentator.
He has called for dramatic policy shifts, and under his leadership Chega won 7% of the vote in the 2022 legislative elections.
Polls
suggest that it could now double this score, leaving it potentially
able to keep a minority centre-right government in power - at a price.
The
leader of the centre-right Democratic Alliance, Luís Montenegro, has
said he would not seek Chega's support for his programme of tax cuts and
free-market reforms.
The
Socialists are led by Pedro Nuno Santos, a former minister under Mr
Costa, who has defended the outgoing government's record.
In eight years of Socialist rule, unemployment has dropped and the economy grew by 2.3% last year.
A total of 10.8 million people are registered to vote in the elections to choose 230 members of parliament.
