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Opposition leader Peter Magyar has emerged as the biggest threat to Orban's grip on power.
Voting in the election for the 199-seat parliament starts at 6 a.m. local time (0400 GMT) and is due to close at 7 p.m. Around 8 million Hungarians are eligible to vote on Sunday.
Opinion polls have shown Orban's Fidesz party trailing Magyar's opposition Tisza party.