Originally Posted by Kacee

This generalized commentary is not entirely accurate and not particularly helpful here (especially since OP's question related to middle name, not suffix).

First, and most importantly, the problem is most assuredly with the airline, not immigration. Because if the airline doesn't like the name match, it won't let you check-in or board the flight.

Further, airlines can differ significantly in how strict they are on middle names. LX expressly states on their website that they don't care whether your middle name is on the ticket. By contrast, for a while, CX was denying boarding to pax who had not listed their middle name when booking the ticket - a particular problem for those who booked using BA Avios, since the BA website at the time did not make any accommodation for middle names.

As review of this thread confirms, it would be very surprising if UA were to deny boarding based on a missing middle name. But it will definitely preclude use of TSA Pre, which requires an exact match.