The criminal investigation into Federal Reserve Chair Jerome Powell threatens to upend the contest over whom President Trump will choose to succeed him as it enters its final stretch. The episode is creating new obstacles on Capitol Hill and raising hard questions about whether any nominee can be seen as independent—tension that was always present but is now much harder to ignore. Trump has made clear he prizes loyalty in his pick, but the Justice Department probe—which Powell said was part of a pressure campaign to get the Fed to lower interest rates—threatens to make that quality a liability. The backlash on Capitol Hill could force that pick to walk a tightrope in any confirmation process this spring. The nominee would have to avoid upsetting Trump with statements that question his efforts to challenge the Fed without provoking concerns from lawmakers or market participants that they are too close to Trump to be credible.