Railing against the right-wing government that until recently counted him as a trusted insider, Hungary’s most popular and improbable opposition leader stood on the back of a flatbed truck surrounded by cheering followers.“Step by step, brick by brick, we will take back our country,” shouted the opposition leader, Peter Magyar, reciting what, during a tour of towns and villages across Hungary, has been the crowd-pleasing mantra of the 43-year-old’s upstart political movement.Making the moment all the more provocative was the location that Mr. Magyar chose for the rally: just down the road from a house that Prime Minister Viktor Orban owns in Felcsut, the village where the leader grew up.Since taking power 14 years ago, Mr. Orban has won four general elections in a row, refashioning Hungary into an “illiberal democracy” often more in tune with China and Russia than with its nominal allies in NATO and the European Union. Now, for the first time in years, the country has been swept by a sense that change, though not imminent, is possible.In elections this month for the European Parliament, Mr. Magyar’s two-month-old party, Tisza, won 30 percent of the vote in Hungary, eclipsing established opposition groups and contributing to the worst performance in years for Mr. Orban’s governing party, Fidesz. Mr. Orban’s party still came in first, but its 44 percent of the vote was down sharply from its tally in previous elections.Mr. Magyar is vague on specific policies beyond lambasting Mr. Orban and his cronies over corruption, particularly the misuse of billions of euros in European Union funding, and Hungary’s tilt toward Russia. “Anybody who knows Hungarian history knows that we were attacked many times by Russia,” he said in an interview.But it remains to be seen whether he can maintain his dizzying momentum until Hungary holds its next election for the national Parliament in 2026.Andras Banko, a 46-year-old entrepreneur who attended Mr. Magyar’s rally in Felcsut, about 25 miles west of Budapest, two weeks before the elections for the European Parliament, said he was not expecting Mr. Orban to lose his grip quickly, given Fidesz’s control of the media and its entrenched patronage networks. But Mr. Magyar, as a conservative, offers the first viable alternative in many years, Mr. Banko noted.“It will take time, but I am fed up having to apologize for my country because of Orban,” he added.He dressed for Mr. Magyar’s rally in a T-shirt mocking the village’s former mayor, Lorinc Meszaros, a friend of Mr. Orban’s and a former pipe fitter who is now a wealthy tycoon.Among the state contracts thrown Mr. Meszaros’s way were ones for work on a nearly 4,000-seat soccer stadium in Felcsut, which has fewer than 2,000 residents. Mr. Orban is a big soccer fan.Mr. Magyar parked his truck near the stadium and pointed to the hulking arena during his speech as an example of why, according to Transparency International, Hungary ranks as the most corrupt country in the European Union.“This country is not for oligarchs. It is for you,” Mr. Magyar shouted to applause.Mr. Magyar has clearly rattled his former allies. He has faced a barrage of abuse from Fidesz and from the media it controls, which have assailed him as an abusive husband, a crook and a traitorous demagogue.“I’m attacked constantly by the government and by the opposition. That is my success,” Mr. Magyar said.Until February, Mr. Magyar, a Fidesz member for more than two decades, was just another Orban apparatchik.His biography featured diplomatic postings in Brussels; executive positions at Fidesz-controlled companies; a friendship with Mr. Orban’s chief of staff; and a failed marriage with Judit Varga, a Fidesz star named justice minister in 2019. Hardly anyone knew his name outside a narrow elite circle in Budapest.Now, he is a national celebrity, reviled by both Fidesz, from which he is trying to attract conservative voters, and by a leftist opposition bloc whose supporters he also wants. He won backing from both groups in the European elections.Mr. Magyar drew only a few hundred people, mostly from out of town, to the rally in Felcsut, held on May 24. But getting even that many was an achievement in a district where loyalty to Fidesz and gratitude for its largess is strong. Rallies in big cities have drawn tens of thousands.His events combine the zeal of a religious revival meeting with a blunt political message: that Mr. Orban, 61, and his shopworn opponents in entrenched opposition parties have been around too long.Mr. Magyar, who dangles a small wooden cross from a wristband, ends gatherings by playing a 19th-century patriotic song and encouraging attendees to hold the hands of their neighbors and raise them in a gesture of solidarity.The fervor, said Csaba Lukacs, managing director of Magyar Hang, or Hungarian Voice, a conservative weekly that is at odds with Fidesz, showed that “there is a huge need for something new in Hungary — to replace the corrupt and or incompetent opposition on the one hand and to overthrow Fidesz on the other.”Fidesz has faced a challenge from a fellow conservative before.For a parliamentary election in 2022, opposition groups across the political spectrum put forward a right-wing provincial mayor as their standard-bearer. That effort flopped, ending in a landslide victory for Fidesz after it deployed its media machine to smear the mayor, Peter Marki-Zay, as a warmonger intent on sending Hungarians to fight against Russia in Ukraine.Mr. Magyar avoids talking about Ukraine. His former allies, ignoring the fact that he spent nearly all his adult life as a member of Fidesz, have nonetheless denounced him as a leftist menace bent on war. (Mr. Magyar says he joined Fidesz in 2002 when it was a “pro-European, pro-NATO, liberal but right-wing party.”)The attacks, said Peter Kreko, director of Political Capital, a Budapest research group, showed that Mr. Magyar had unnerved the government.“He is the guy everybody is suddenly talking about — in the pub, on the bus, at hairdressers, everywhere,” Mr. Kreko said.Agoston Mraz, director of the Nezopont Institute, which conducts opinion polls for Fidesz, acknowledged that the party had targeted Mr. Magyar but said the charges were “not just lies but based on real stories” about him.Mr. Mraz cited an interview given to a tabloid by Ms. Varga, the mother of Mr. Magyar’s three children, in which she described him as an emotionally abusive narcissist. The couple divorced last year.The attacks, Mr. Mraz added, “have worked,” blunting Mr. Magyar’s appeal to core Fidesz voters and made sure that he “is now mostly dangerous for the opposition.”Agnes Vadai, a member of Parliament and deputy leader of the left-leaning Democratic Coalition, said she agreed. She dismissed Mr. Magyar as an “unhealthy phenomenon” who would mostly hurt Mr. Orban’s foes by attacking them as well as the prime minister. “If you want to overthrow the system, you don’t attack the opposition,” she said.Mr. Magyar said he had been moved to act by disgust over Mr. Orban’s handling of a giant political scandal set off by revelations this year that a man convicted of covering up sexual abuse at a state-run children’s home had been pardoned.Amid the uproar that ensued, Mr. Orban forced the resignation of Hungary’s president, Katalin Novak, and of Ms. Varga as leader of Fidesz’s election campaign for the European Parliament.“They wanted to push the whole scandal on to my former wife and the president and away from the prime minister,” Mr. Magyar said. He added that he had been “astonished” by subsequent claims by Ms. Varga of emotional abuse.While perhaps solidifying the governing party’s base, the vitriol against Mr. Magyar by Fidesz-friendly media outlets has fallen flat with the unaffiliated.Sandor Szarvas, a voter in that category, turned out to hear Mr. Magyar speak in Bicke, a small town near Felcsut. The children’s home in Bicke was at the center of the pedophilia scandal.“We don’t eat food from the toilet, so we obviously don’t get news from Fidesz media,” Mr. Szarvas said.