BUDAPEST—A landslide victory has left Hungary’s nationalist Prime Minister Viktor Orban in a position to cement both his authority at home and his influence in the European Union, potentially shifting the Continent’s balance of power further to the right, even as international observers declared the vote unfair.
Mr. Orban’s Fidesz party won 134 of the legislature’s 199 seats in Sunday’s election, ensuring it has the number of votes to pass major overhauls and constitutional changes unilaterally. Hungarian political analysts...