Opposition hopes of an upset are pinned on a potential surge in turnout due to simmering displeasure with the 54-year-old premier, in power since 2010.
A mainly first-past-the-post election system that was designed by Fidesz favours it over the fragmented opposition parties, but polls show around 40 percent of the eight-million-strong electorate are undecided.
Polls also show a slight majority of Hungarians favour a change in government, while civil groups have urged nationwide tactical voting to foil Fidesz candidates.
"There is anger in the air, and danger for Orban if people turned off by politics in recent years vote en masse," an analyst with Policy Solutions, Andras Biro-Nagy, told AFP.
A clear win for Fidesz is still widely seen as probable and would cement Orban's plan, declared in 2014, to mould ex-communist Hungary (population 9.8 million) into an "illiberal" state.
It would also keep Orban at the forefront of the deepening division between the European Union's western and eastern members.
Since the 2015 migration crisis Orban has cast himself as a defender of national sovereignty and "Christian Europe" against the "globalist elite".
His tough anti-immigration stance and verbal onslaughts against "Brussels bureaucrats" have gained him followers in central Europe, especially nearby Poland as well as far-right circles in western Europe and beyond.
Brussels has hit back by suing Budapest over its refusal to join the EU refugee resettlement plan, as well as laws targeting civil society groups and a university linked to the liberal US billionaire George Soros.
If Fidesz loses on Sunday "an internationalist government" backed by Soros, whom Orban accuses of orchestrating mass migration, will turn Hungary into a "country of immigrants", the prime minister said last week.
An Orban win on the other hand will see his rhetoric turn "increasingly confrontational," an analyst with Political Capital, Edit Zgut, told AFP.
"And Orban will likely become an even stronger point of reference for European radical-right forces," she said.
During the election campaign Orban has stepped up his long-running attacks on the Hungarian-born Soros which have included media blitzes called xenophobic and even anti-Semitic by detractors.
Fidesz posters show the 87-year-old Jewish financier with opposition leaders brandishing wire-cutters ready to take down anti-migrant border fences Orban erected in 2015.
Since sweeping into power with a two-thirds majority eight years ago, Orban has transformed Hungary, overhauling its state institutions and constitution.
In recent years, his government's tax policies have benefited an expanding upper middle class while the economy, powered by German car-makers and EU budget transfers, has been picking up.
"We need four more years in power to make our achievements irreversible," Orban said in November.
But critics at home and abroad, including in Brussels and Washington, claim that Orban's reforms have eroded democratic institutions like the judiciary and the press, and that his clampdown on civil groups apes tactics used by Russian President Vladimir Putin, a regular visitor to Budapest.
"Hungary is drifting away from Europe toward the Russian sphere of influence," the Socialists' Karacsony told AFP.
Last month Orban pledged to take "moral, political and legal retribution" on opponents after the election, an apparent threat that alarmed opposition parties.
While the Fidesz election campaign, anchored by the slogan "For us, Hungary is first!" has focused solely on high-pitched warnings about Muslim and African migrants, Orban himself has shunned debate with rivals and appeared in public only at carefully choreographed events.
His aura of invincibility has been shaken though by a wave of recent corruption scandals involving close allies and family members that have heightened unpredictability about Sunday's result.
After a bruising campaign analysts say Fidesz is now unlikely to secure a third consecutive two-thirds majority in the 199-seat assembly, but would only lose an absolute majority if turnout tops 70 percent.
"Whoever does not vote Sunday is voting for Orban," said Jobbik's Vona.
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