Brazil's top electoral authority said on Tuesday that it would re-open an investigation into alleged misuse of funds during President Dilma Rousseff's re-election campaign.
The Supreme Electoral Court will try to determine if Ms Rousseff's and Vice-President Michel Temer's campaign drew on donations from illegal sources.
The probe was requested by the opposition PSDB party.
It comes at a time when Ms Rousseff's approval rating is at a record low.
If it were to uncover irregularities, the court could invalidate Ms Rousseff's and Mr Temer's election and trigger fresh polls.
But experts say in order for that to happen the irregularities would have to be so serious that the judges deem them to have endangered the legitimacy of the election.
Moreover, the inquiry is likely to take many months, if not years, and a fresh election is therefore an unlikely scenario, electoral law experts said.
The case was shelved in February when a judge ruled there was not enough evidence to proceed, but the PSDB appealed against the decision.
The opposition party alleges that there are indications that construction firms involved in a massive corruption scandal donated money to Ms Rousseff's campaign.
The construction firms allegedly bribed politicians and the state oil company, Petrobras, to secure contracts.
The court ruled five to two to re-open the investigation.
Ms Rousseff's popularity has fallen to single digits since her re-election last year, as the economy has gone into recession and corruption scandals involving members of her governing Workers' Party have widened.
In August, hundreds of thousands of people took part in protests across Brazil calling for her impeachment.
There have also been demonstrations backing Ms Rousseff.