Conservative member of Parliament Boris Johnson, who campaigned to leave the EU., is the clear favorite to replace May among bookmakers and party members, though he faces several challenges. Trump described Johnson, a former mayor of London, as a friend during his visit to the U.K. in 2018. The U.S. president also said Johnson would make a good prime minister, comments that irritated May at the time and for which he apologized. The delayed Brexit process and the widespread hatred of May’s deal triggered a surge in support for the populist leader Nigel Farage’s Brexit Party, which is set to...