Hungarians cast their ballots in a national election Sunday, a test of whether Prime Minister Viktor Orban —a fierce nationalist and leading figure on the European right—will retain his lock on parliament following a campaign in which he denounced immigrants and international institutions.

Mr. Orban, seeking his fourth term in power, is heavily favored to win, though projections wavered in recent weeks, as corruption scandals and last-minute attempts to unite Hungary’s divided opposition led some analysts to weaken their predictions of an incumbent victory.

Mr. Orban’s party Fidesz is facing more than 20 different rivals on the ballot, ranging from centrist Politics Can Be Different faction to the ultranationalist Jobbik party.

The Hungarian leader becoming a hero to Europe’s nativist movements, the signature accomplishment he hopes will give him a landslide victory similar those he won in 2010 and 2014. A series of constitutional changes favor his party: Fidesz helped draw the voting districts, and eliminated runoffs, giving the party an edge over its fractured opponents. Polls project it to get close to 50% of the vote.

A government billboard shows George Soros and opposition party leaders in Budapest. The billboard reads: "Stop Soros Candidates!” PHOTO: BERNADETT SZABO/REUTERS

Still, Mr. Orban’s party risks losing its hold in parliament, where it controls 132 of 199 seats, and for the first time in years, the prime minister—who will have led Hungary for half of its post-Communist independence if he serves another four years—appears vulnerable. A by-election in February produced a shock, landslide defeat for his party in a small town where it had last won by a 44-percentage-point margin.

“The narrative was Fidesz was this undefeatable bloc, you can’t do anything to stop them,” said Daniel Berg, a candidate for the country’s Momentum party. “The narrative has changed.”

In response, Mr. Orban has doubled down on a strategy that has carried him to victory in the past. Billboards warn that millions of Africans and Middle Easterners will invade Hungary if Mr. Orban loses.

Sunday’s vote brings a test of whether those antiforeign themes continue to resonate in a country that has been bombarded with messages against immigrants since Europe’s refugee crisis exploded in 2015. There are signs that Hungarians outside of Mr. Orban’s base of about 2 million voters have wilted on the message. The founder of his party, Zsolt Bayer, has publicly called on Mr. Orban to tone down the campaign and find new issues.

Voters haven't shown the same alarm on immigration that they did in 2015: A 2016 referendum on migration inspired so few people to turn out that its results weren’t legally binding. Anti-migrant billboards in the capital, Budapest, have been defaced.