Bernie Sanders was at it again — center stage, his baritone building to something — right hand raised and restless, as if he were scribbling his words on an invisible chalkboard.

“Medicare for All is comprehensive,” he insisted, swatting away concerns that his health care plan might imperil benefits for union members. “It covers all health care needs,” he said. Older Americans, he enthused, would be covered for dental care, hearing aids, eyeglasses.

[Here’s what to watch for in Wednesday night’s debate.]

“You don’t know that,” Tim Ryan, a union-country congressman from northeast Ohio, interjected, later reasoning that Mr. Sanders “does not know all of the union contracts in the United States.” “You don’t know that, Bernie.”

Mr. Ryan turned to face him. For a moment, Mr. Sanders appeared inclined to ignore the interruption. “Second of all, second of all,” he said, waving off Mr. Ryan. Then the Vermont senator reconsidered.