Brazil’s President Dilma Rousseff is digging in for a fight, and she could embrace populist-themed economic policies in a bid to cling to power. Pictured: Rousseff at a meeting of BRICS leaders, in Ufa, Russia, July 9, 2015. Ria Novosti via Getty Images/Host Photo Agency/Vitaliy Belousov

SAO PAULO (Reuters) - Brazil's federal police have opened a preliminary investigation into alleged irregularities in President Dilma Rousseff's re-election campaign last year.

According to the media office of the federal police, the probe was opened on Oct. 7, following a decision by Justice Gilmar Mendes of the Superior Electoral Court to look into allegations of wrongdoing by the Rousseff campaign. Mendes, also a justice at Brazil's top court, recently asked prosecutors to investigate the campaign.

Efforts to contact staff who worked in the campaign for comment were unsuccessful. Rousseff's office declined to comment on the probe.

The electoral court, known as TSE, voted 5 to 2 on the decision last week. An investigation aims to find out whether Rousseff and her running mate, Vice President Michel Temer, abused their power while in office to run the campaign and whether illegal money was used as funding.

The opening of the police probe could add fuel to opposition attempts in Congress to impeach Rousseff. Those efforts lost traction earlier in the week after Rousseff and Lower House Speaker Eduardo Cunha forged an uneasy truce as both face the risk of being ousted from office.

If found guilty, Rousseff's victory last October could be invalidated, though the judicial case is expected to last for months if not years and can be appealed to the Supreme Court.