Donald Trump has endorsed Hungary’s prime minister, Viktor Orbán, who faces his toughest electoral challenge next month since taking power 16 years ago, as Europe’s far-right leaders gather for a “grand assembly” in Budapest.

In a video message, the US president told the national-conservative Cpac Hungary conference in the capital on Saturday that Orbàn, who has been trailing in the polls behind a centre-right rival for more than a year, was a “fantastic guy”.

Trump, who also backed Orbán on social media last month, said he had been a strong leader who had “shown the entire world what’s possible when you defend your borders, your culture, your heritage, your sovereignty and your values”.

“I hope he wins, and I hope he wins big,” he said.

Orbán responded that the west had become a better place since Trump returned to power, with progressive policies being rolled back and traditional family and Christian values restored.

Polling averages suggest Orbán’s challenger, Péter Magyar and his Tisza – or Respect and Freedom – party, could outscore him by between nine and 11 percentage points on 12 April in what is likely to be Europe’s most consequential parliamentary election of the year.

Several leading European far-right figures, including Santiago Abascal of Spain’s Vox, André Ventura of Portugal’s Chega, Martin Helme of Estonia’s Ekre and Mateusz Morawiecki of Poland’s Law and Justice party, attended the weekend event.

They will be joined on Monday by Marine Le Pen of France’s National Rally, Matteo Salvini of Italy’s League and Geert Wilders of the Dutch Freedom party for a “Patriots’ Grand Assembly”, named after their group in the European parliament.

Orbán has long been at loggerheads with the EU over a range of issues. In defiance of Brussels, he has maintained cordial ties with Moscow, refuses to send weapons to Ukraine, and says Kyiv can never join the EU.

Poland’s prime minister, Donald Tusk, said on Sunday that he was not surprised by a Washington Post report last week that alleged Russia’s foreign intelligence service had proposed staging an assassination attempt against Orbán to boost his chances.

The report also said Hungary’s foreign minister, Péter Szijjártó, had called his Russian counterpart, Sergei Lavrov, during EU summits to brief him. Tusk said it should not come as a surprise that Hungary leaked “every detail” of EU summits to Moscow.

Szijjártó described the allegations as “mad conspiracy theories” that were part of an international smear campaign intended to influence next month’s election.

Many of the far-right leaders scheduled to attend Monday’s gathering were among nearly a dozen who endorsed Orbán in a campaign video released in January. In it, Alice Weidel of Alternative für Deutschland said: “Europe needs Viktor Orbán.”

Media reports had previously suggested that the US vice-president, JD Vance, would attend the Budapest gathering, but Szijjártó said last week that the visit would take place in early April instead.