Even if Fidesz does gain its expected parliamentary majority, analysts will be watching to see whether it falls short of the two-thirds "supermajority" that has enabled it to pass some of its most controversial bills. Firebrand Prime Minister Orban has clashed repeatedly with other European Union leaders during his last term, especially with regard to the bloc's response to the refugee influx across Europe in 2015 and 2016.
Orban's right-wing Fidesz party and its ally the Christian Democrats are projected to win a majority, AP reports. But the high turnout and Hungary's complex voting system complicated more exact predictions.
Rivals say that leaves open the possibility of an upset and are urging voters to turn out in record numbers.
Opposition parties have not coordinated this closely on a national level, but tactical voting could nevertheless represent a danger to Fidesz in 30-40 "swing seats".
"Today will decide whether Hungary becomes an emigrant country or not - and I wouldn't like Hungary to be an emigrant country", Mr Vona said.
The anti-immigrant campaign has gone down well with around two million core voters of Fidesz. As Radio Liberty writes, according to official data, as of 15:00 local time, more than 53.6% of voters voted - this is the highest turnout rate for this hour since 2002. That was the highest turnout figure at that time since at least 1998. "I am asking everyone to take part in the election".
A voter approaches ballot boxes at a polling station in Budapest. A defeat could open the way for a disparate group of opposition parties to roll back some of his self-styled illiberal measures, some of which were modeled on Russian Federation.
"This is one of the most unpredictable elections", said Attila Juhasz, an analyst at Political Capital in Budapest.More news: Art Dealer Killed in Trump Tower Fire
Vona said: "I feel a surprise and a Jobbik breakthrough can be expected in the election".
"We are convinced that high turnout definitely reflects. that people want a change in government", Socialist spokeswoman Bernadett Budai was quoted as saying by national news agency MTI.
"Unfortunately, for those on the right who are disappointed with Orban, there is no democratic conservative alternative like Germany's CDU or a French Macron-type party", said Paul Lendvai, a journalist and author of "Orban: Hungary's Strongman".
"We love our country and we are fighting for our country", Orban said.
After casting his vote in a wealthy district of Budapest with his wife, Orban said Hungary's future was at stake in the vote.
Additionally, Hungary has backed Poland in its rule of law battle with Brussels, while remaining outspoken on European Union policies, including immigration.
Asked whether he was fighting the EU he said: "The EU is not in Brussels".