Opinion polls indicate Orban, who has clashed repeatedly with Brussels and has friendly ties with Moscow, could be ousted in Sunday's vote after 16 years in power.
Opposition leader Peter Magyar has attracted strong support as anger rises among Hungarians over the struggling economy.
"Many millions of Hungarians will vote... for a European, functional, humane, free and independent Hungary," Magyar said to a rally of some 10,000 supporters.
Magyar accused Orban of aligning Hungary with Moscow, saying his own government would firmly re-anchor the country within Europe.
"Hungary's place is, was and will be in Europe," he said.
Some eight million citizens are eligible to cast their ballots. Polling booths open at 6am on Sunday.
Orban received last-ditch social media support from neighbouring allies on Saturday.
"I have never met such a warrior for sovereignty and national interests of one's country as Hungarian Prime Minister Viktor Orban," Slovak Prime Minister Robert Fico said.
Under Fico, who is in power for a fourth time since 2023, Slovakia has been a key ally for its neighbour Hungary, with both keeping warm relations with Moscow, opposing European Union sanctions and continuing to buy Russian oil and gas.
Both have clashed with European Union institutions over the rule of law.
Czech populist Prime Minister Andrej Babis, a billionaire businessman who has turned from a liberal pro-EU politician into a close Orban ally in their Patriots for Europe faction in the European Parliament, also voiced his support.
"He (Orban) has always fought for a stronger Europe, one built on peace, sovereign nations, sovereign member states, competitiveness," Babis said on X.
"In turbulent times, choosing stability and proven leadership matters more than ever."
Since Babis returned to government last year after a stint in opposition, the Czech Republic has slashed aid to Ukraine and, following Hungary and Slovakia, refused to participate in the EU's 90 billion euro ($A149.35 billion) loan for Kyiv.
The Czech position on Russia has, however, remained more mainstream than those of Hungary and Slovakia.
Babis's cabinet, which includes a far-right anti-NATO party, is trying to reverse EU decarbonisation policies and is preparing legislation to revamp public media and bring non-governmental organisations under closer scrutiny.
His opponents say the plans are modelled on Hungarian and Slovak reforms that undermine democratic standards.
with DPA
