The strain of the long campaign and a frenetic transition period is beginning to wear on the face of President-elect Barack Obama, who has developed a facial tic under his right eye.

The tic on the lower part of his right orbital bone is clearly visible in his recent interview with ABC’s Barbara Walters. Campaign insiders say it first emerged during the primary season and has now become chronic.

A facial tic is a repetitive, spasmodic movement often involving the eyes and facial muscles. The cause of tics is unknown, but stress appears to increase their severity.

“The patients I’ve treated with tic disorders had one thing in common: They knew that the tics worsened when they were under stress,” said Dr. Robert T. London, a psychiatrist with the New York University Medical Center…