Following an acrimonious campaign in which the rightwing nationalist prime minister projected himself as a savior of Hungary's Christian culture against Muslim migration into Europe, all opinion polls had put his Fidesz party well ahead.
Fidesz lawmaker Gergely Gulyas told private broadcaster ATV his party was unlikely to retain its two-thirds parliamentary majority.
Voters in Hungary are going to the polls in a general election on Sunday.
Analysts say a high turnout favors the opposition, especially Jobbik.
"Only a dramatic outcome of the election would force a significant shift in the direction of policymaking", Barclays said in a note.
But as Chairman of the Party, Vona has gradually brought Jobbik into the mainstream in the hope of widening its political base.
Uncertainties about Mr Orban's margin of victory are caused by Hungary's electoral system in which voters cast two ballots, one for an individual candidate in their region and another for a party list.
Prime Minister Viktor Orban's Fidesz party is expected to win the majority of the 199 parliamentary seats, with Vona's Jobbik and a left-wing alliance of the Socialist Party and the Dialogue party led by Gergely Karacsony considered the leading challengers.
A strong win for Orban would also boost other right-wing nationalists in Central Europe, in Poland and in neighboring Austria, and expose cracks in the 28-nation EU.
Opposition parties have lacked close coordination on a national level, but tactical voting could nevertheless represent a danger to Fidesz in 30-40 "swing seats".
And if migrants settle in Hungary, Orban claims Hungary's economic development will end, its support for rural areas will dwindle, women and girls will be "hunted down" and Budapest, the capital, will become "unrecognizable".
"Migrants should not come here, and this (Fidesz') economic policy should continue", said Janos Dimovics, a pensioner who came from his village to see Orban.
"Safety is first", said Julia Scharle, 27, holding her child outside the voting district where Orban cast his vote.
For rebellions such as refusal to meet EU requirements on accepting mass migration, accusations of violations of university freedom, freedom of assembly and association, social rights, and alleged "everyday corruption", the European Union could stop money going to Hungary, or even insist Hungary repays funds already given.
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.