LONDON — Jacob Rees-Mogg, the leader of Britain’s House of Commons and a prominent ally of Prime Minister Boris Johnson, suggested this week that victims of the 2017 Grenfell Tower fire lacked the common sense to defy firefighters’ orders and flee the building, costing them their lives.

The comments, appearing to lay some blame for the 72 deaths on the people trapped in the flames, angered other lawmakers and shocked even critics who have grown used to Mr. Rees-Mogg’s flashes of aristocratic obliviousness. They also threatened to undercut the Conservative Party’s main election pitch — that it’s a party for working-class voters alienated from the Labour Party over Brexit — just as campaigning for the Dec. 12 vote gets underway.

Nicknamed “the honorable gentleman from the 18th century,” Mr. Rees-Mogg was recently seen reclining on the front bench of the House of Commons, making clear his boredom during a fierce Brexit debate. And lawmakers said that a speech he gave in the run-up to a crunch vote was so arrogant that it persuaded members of his own Conservative Party to change their minds and rebuff the government’s Brexit plans.

Mr. Johnson, who is about to formally launch the Conservative campaign, is already saddled with a reputation for being haughty and occasionally out of touch. For a government trying to dispel an image of being a bunch of smug, entitled private-school types sleepwalking into national crisis, Mr. Rees-Mogg’s air of condescension and disdain has not been helpful.