While there is little doubt that his 'No' camp will comfortably win, the poll could still end in embarrassment for Orban if it fails to reach the required 50-percent turnout and is deemed invalid.
To avoid this, the right-wing government has led a fierce media offensive urging the eight-million-strong electorate to spurn the EU deal, which seeks to share migrants around the 28-member bloc via mandatory quotas without the consent of national parliaments.
Orban warned yesterday that mass migration was a "threat... To Europe's safe way of life" and that Hungarians had "a duty" to fight the failed "liberal methods" of the "Brussels elite".
"We can send a message to each European... Telling them that it depends on us, European citizens, to bring the EU back to reason, with common effort, or let it disintegrate," he wrote in the Magyar Idok newspaper.
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The EU proposal - spearheaded by Germany and approved by most EU countries last year - is aimed at easing pressure on Italy and Greece, the bloc's main entry points for hundreds of thousands of people mainly fleeing war in Syria.
Hungary has not accepted a single refugee allocated under the scheme and instead joined Slovakia in filing a legal challenge against it.
The referendum threatens to further split the quarrelling bloc, already weakened by its worst migration crisis since 1945 and Britain's decision in June to leave the union.
"If referendums are going to be organised on every decision of the Council of Ministers and the European Parliament, legal security is in danger," EU President Jean-Claude Juncker warned in late July.
As anti-migrant parties surge in popularity across the continent, Orban has emerged as the populist standard-bearer of those opposed to German Chancellor Angela Merkel's "open-door" policy.
Opposition parties and rights groups held protests ahead of the vote, accusing Orban of whipping up xenophobia despite the lack of asylum-seekers in the country.
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