“Anyone who saw that press conference would have been reminded of his limitations as a communicator,” said Tim Bale, a professor of politics at Queen Mary, University of London. “Sometimes a crisis can see a caterpillar turn into a butterfly, but I rather doubt that will happen here.”

As the first secretary of state in Mr. Johnson’s government, Mr. Raab has formally been his deputy since Mr. Johnson won leadership of Britain’s Conservative Party last year. That left little doubt that he would be the government’s “designated survivor,” the stand-in should Mr. Johnson fall ill.

But his responsibilities have grown far more quickly than anyone expected. He has already led meetings with the powerful heads of each facet of the government’s coronavirus response this week. And he will lead cabinet meetings and make decisions on Mr. Johnson’s behalf if the prime minister cannot do so himself from intensive care.

Mr. Johnson’s office has said the prime minister is still breathing on his own and has not been put on a ventilator. But his prognosis remains uncertain, and the government has been cagey about his condition since he tested positive for the virus last month.

During his decade as a Conservative lawmaker, Mr. Raab’s most influential moment may have come in 2018 when he resigned from the cabinet of Mr. Johnson’s predecessor, Theresa May, after only four months in his job.

A hard-line Brexit backer, he resigned in protest of the draft withdrawal agreement that he had ostensibly helped to negotiate as Mrs. May’s Brexit secretary. In truth, he had only limited influence over the deal, which envisioned Britain keeping some distant ties to the European Union after Brexit. But he became part of a parade of pro-Brexit lawmakers angry at Mrs. May’s plan who helped force her to step down as Conservative Party leader and prime minister.