Keir Starmer did what he could on Monday to weather a parliamentary showdown on his handling of the Peter Mandelson scandal. On Tuesday, it may rest with a bureaucrat little known outside Westminster to decide his fate.The U.K. prime minister is halfway through two bruising days of public testimony on his catastrophic decision to appoint Mandelson as U.S. ambassador, a call which is threatening to bring about his downfall. A committee appearance by Olly Robbins, whom Starmer last week fired as the Foreign Office’s top civil servant, now presents the most perilous moment of Starmer’s premiership so far. Compounding the danger, the main opposition Conservative Party said they’d secured an emergency debate on the matter as 'the first order of public business” in the House of Commons on Tuesday.Facing 2½ hours of questions in the Commons chamber on Monday, Starmer insisted that neither he, nor any minister or member of his office — not even the then-head of the civil service Chris Wormald — had been informed that Mandelson’s appointment was opposed by security vetting officials.