LONDON — Not long after asking the House of Commons to “contemplate the current constitutional confusion and consider the chaos this concatenation of circumstances could create” on Tuesday night, the British lawmaker Jacob Rees-Mogg reclined on the front bench of the chamber until he was nearly horizontal.

Around him colleagues raged at the power grabs of Prime Minister Boris Johnson’s government, in which Mr. Rees-Mogg serves as leader of the House of Commons. But there was Mr. Rees-Mogg, nicknamed “the honorable gentleman from the 18th century” for his old-world tics and antiquarian poshness, looking like a Degas model, with his willowy frame and long, angular face.

His legs were crossed. His midsection, wrapped in a double-breasted suit, was nearly flush with the green leather bench. He clasped his hands and tipped back his head.