It was "a last-minute and accidental arrival through the back door," Mr. Puigdemont recalled in an interview this week at the Gothic palace of his regional government in Barcelona.

Now, as Catalonia attempts to hold an independence referendum on Sunday, Mr. Puigdemont, 54, sits at the heart of a constitutional crisis for Spain, an insurgent in the eyes of Madrid.

If the vote goes on, as he says it will, Mr. Puigdemont (pronounced POOTCH-da-mon) could be barred from politics and go to prison for misusing public money to hold a referendum that Spanish courts have ordered suspended.

The prospect seems to leave him unbothered. Mr. Puigdemont may be an accidental leader, but he is a purposeful proponent that Catalonia — prosperous and distinct in culture, history and language — should be independent.

A former journalist, with a Beatles mop-top haircut, he was already calling for separation from Spain on the streets of Barcelona in the early 1980s, when secessionism was a marginal movement in Catalonia.