While France bars contributions from businesses, loans are allowed. A Russian bank made headlines in recent years after lending millions of euros to the far-right National Front party of Marine Le Pen. After that bank failed last year, the party complained that it had been shunned by French banks and declared itself in the market for a new lender.

If nothing else, European far-right parties are gaining newly emboldened allies.

“I expect the Trump administration to be more open to these parties than Obama, certainly,” said Representative Steve King, an Iowa Republican who is an ally both of President Trump and the European far right, having met with various party leaders during a recent European trip.

The State Department, in a statement, declined “to comment on political parties in foreign elections.”

Mr. Horowitz, who has long sounded alarms on Muslim immigration, first rallied to Mr. Wilders’s side after the Dutch politician was put on trial in 2010 for inciting hatred against Muslims with a film he made that attacked the Quran; he was acquitted the next year. Mr. Wilders was more recently found guilty of incitement after leading an anti-Moroccan chant at a rally, though he avoided a fine.

“I think he’s the Paul Revere of Europe,” Mr. Horowitz said in an interview. “Geert Wilders is a hero, and I think he’s a hero of the most important battle of our times, the battle to defend free speech,” he added, calling the situation in Europe a “nightmare.”

Though Mr. Horowitz’s donations adhere to Dutch standards, there was some question of whether they comply with American law.