BERLIN — Prosecutors have begun an investigation into a leader and three members of the far-right Alternative for Germany Party for accepting “dubious donations” worth $145,000 from a Swiss company in the heat of last year’s election campaign, which saw the party surge to the third-strongest in the country.

The main target of the investigation is Alice Weidel, who was a leading candidate in last year’s national election and has since served as its floor leader in Parliament. The investigation could undermine the party, known by its German initials AfD, which has become the main opposition force in Parliament.

So far AfD has stood behind Ms. Weidel, who has denied the campaign finance violations, which were first uncovered last week by the Sueddeutsche Zeitung newspaper and the public broadcasters NDR and WDR. Prosecutors in the southern city of Konstanz then started the investigation, but had to wait for Ms. Weidel’s parliamentary immunity to be lifted.

Under German law, political parties are banned from accepting donations of more than 1,000 euros, about $1,100, from anyone living in a country that is not a member of the European Union, unless the donor is a German national. The full identity of the individual donor must also be stated, not just the name of an entity like a company or foundation.