Borissov has effectively played the “good European,” said Ruslan Stefanov, director of the economic program at the Center for the Study of Democracy, a research group in Sofia. The concern is that oligarchs control critical assets and that leaves the country at the whim of a narrow group of people—what he and his colleagues call a “captured state.”

“If they change their allegiance, it changes the whole orientation of the country,” said Stefanov, who co-authored a study entitled “The Kremlin Playbook in Southeast Europe” published this year.