(Bloomberg) — The tumult of personnel turnover that’s come to characterize Donald Trump’s administration is suddenly posing a major problem — just as his presidency enters an especially risky phase. The president lacks an immediate successor for Chief of Staff John Kelly following his announcement on Saturday that the retired Marine general would leave the White House. Trump’s failure to line up a replacement before abruptly announcing Kelly’s departure to reporters sets up a potentially chaotic transition for a job crucial to maintaining a semblance of stability under a commander-in-chief famed for his unpredictability, Jennifer Jacobs and Margaret Talev write. The president said yesterday evening that he was interviewing chief-of-staff candidates after Vice President Mike Pence’s top aide, Nick Ayers, turned him down. But Ayers’s rejection of Trump’s overtures hints at the challenge whoever assumes the post will face. Kelly’s successor must help Trump deal with the new Democratic House majority — some members of which would like to see the president impeached — as well as the next phases of Special Counsel Robert Mueller’s probe into alleged Russian election meddling. Added to those are the demands of navigating the 2020 reelection campaign. For U.S. allies in Europe, Asia and beyond, Kelly’s departure means there’s one fewer of the so-called adults in the room to constrain Trump.