A 65-year-old patient with heart failure entered his clinic — and urgently needed oxygen, he said. The tanks sat in another room at the ready, he recalled.

But he said his Cuban and Venezuelan superiors told him to use the oxygen as a political weapon instead: Not for medical emergencies that day, but to be doled out closer to the election, part of a national strategy to compel patients to vote for the government.

May 20, 2018, was nearing, he said, and the message was clear: Mr. Maduro needed to win, at any cost.

“There was oxygen, but they didn’t let me use it,” said Dr. Arias, who defected from the Cuban government’s medical program late last year and now lives in Chile. “We had to leave it for the election.”