Opinion polls have consistently put Orban and his right-wing Fidesz party 20 or more points clear of their nearest rivals, Jobbik, a far-right party that has been moving towards the center, and the center-left Socialists.
Around 7.9 million Hungarians are eligible to vote and turnout figures will be closely watched for any clues of a possible upset.
Polls close at 1700 GMT.
The election produced a turnout of around 70 percent, exceeding the past three elections.
Clad in a green summer jacket and white shirt, Jobbik leader Gabor Vona, 39, arrived to vote in the eastern Hungarian city of Gyongyos, his home town and the district where he is likely to win a seat. "This is when all those who want a change of government. ask all those who have yet to vote to by all means go and vote". "Emigration may or may not define Hungary, and I would prefer that it does not".
The EU has struggled to respond as Orban's government has used its two landslide victories in 2010 and 2014 to erode democratic checks and balances, by curbing the powers of the constitutional court, controlling the media, and appointing loyalists to key positions.
Orban himself voted early on Sunday morning with his wife at a school in the leafy Zugliget suburb of Budapest. He is also credited with keeping the budget deficit under control, reducing unemployment and some of Hungary's debt, and putting its economy on a growth track.More news: BJP MPs to observe day-long protest fasts against Parliament's disruption
At a final campaign rally Friday, he urged his voters to turn out in masses on electio day. In Hodmezovasarhely, a Fidesz stronghold in southeastern Hungary, voters complaining of graft, cronyism, and intimidation elected an independent in a February mayoral election for the first time in two decades.
"My little daughter must be my primary concern, to make her future safe". The Socialists came in third with 14%. She would not reveal her voting preference.
In March the government also gave pre-election handouts to millions of families and pensioners.
Despite being tipped to take two-thirds of the 199 parliamentary seats for a third time only months ago, ruling party officials have said that now appears out of reach. In advance of the Brexit vote in June 2016, polls showed that Hungarian voters, second only to Poles, were the most supportive of Brussels in the entire 28-state bloc.
Prime Minister Viktor Orbán's ruling Fidesz party has pointed to the high turnout as a sign that Hungarian democracy is healthy, but the increased turnout could give Orbán something to worry about: numerous voters waiting in line in Budapest were young, and young, city-dwelling Hungarians do not tend to support the prime minister.
His business allies are also expected to expand their economic domains.
"Only a dramatic outcome of the election would force a significant shift in the direction of policymaking", Barclays said in a note.