When António Costa, a prime minister well liked by European leaders, handily won his third term as prime minister in 2022, many Portuguese prepared for a lasting, stable government given his Socialist Party’s strong majority in Parliament.

But by late last year, Mr. Costa had resigned, his government embroiled in a corruption investigation involving lithium exploration concessions.

On Sunday, Portugal faces a new election. It has raised the prospect that the Socialist Party could lose power for the first time in more than eight years, as well as the possibility of an unstable minority government.

One of the most significant changes in the election campaign has been the rise of a hard-right populist party.

What set off the vote?

Mr. Costa resigned in November after the opening of a corruption investigation into lithium exploration concessions, hydrogen production and the construction of a data center.