The conflict was particularly complicated, even emotionally so, with Mr. Tusk’s old friend and anti-Communist colleague, the Hungarian prime minister, Viktor Orban. And he is now even more Mr. Tusk’s problem.

Mr. Tusk has just begun a new job in Brussels as head of a center-right party grouping called the European People’s Party. Mr. Orban and his Fidesz party are prominent members, and Mr. Tusk’s predecessor did very little to rein in Mr. Orban, because the party needed his votes in the European Parliament. So what will Mr. Tusk do?

Mr. Tusk is struggling to answer, because he has seen the transformation of Mr. Orban from an idealistic anti-Communist to a populist authoritarian, dabbling in Islamophobia and anti-Semitism and proud of creating an “illiberal democracy.’’

“I have a problem with Viktor Orban, but also a private one, a personal one,’’ Mr. Tusk said. “Because we have known each other for so many years and our friendship started when he was one of the most talented and promising liberal democrats in our part of Europe,” who stood up to the Communist authorities in 1989.

“When I was president of the European Council he was my partner, and paradoxically, also because of our old friendship, we were personally very loyal to each other,’’ Mr. Tusk said. Then he hesitated, sighed, then said, “This is why it is something painful for me, because I like him as a person.’’

Mr. Orban is smart, Mr. Tusk agreed. “You know this is why I think he is cynical, because he is too intelligent not to understand what is the problem. And of course cynicism is nothing new in politics, but the practical consequences of his cynicism in Hungary are,’’ he stopped again, then said: “I think it went too far. At least it’s my opinion.’’