President-elect Donald Trump and his transition team are zeroing in on how to carry out his promised, largest-ever illegal immigrant deportation plan, but major challenges stand in the way.

Logistical problems, from finding enough airplanes and staffing to searching for people and detaining those in custody, are a top concern for the outside advisers and planners already in government. Implications, including the agriculture industry’s pushback over a possible worker shortage and blowback from Democratic-run sanctuary zones, also threaten to derail the operation before it gets underway.