Why the opposition? The wall is a ridiculous waste of 24 billion USD that as per some studies would slow the transition across the US border of most migrants by 5-10 seconds at maximum. Not to discuss the logistic and legal issues associated, the historical evidence of how walls have worked out in the past when used for this type of issue, the fact that the money could be used in other ways for other more meaningful things, and the fact that there isnt even a basic definition of what type of wall is proposed (we've heard everything from a wall only in philosophy and ideology to a solar paneled layered 100 meter thing spanning from California to Texas)
Currently people tunnel under walls, climb walls, are smuggled in via 18 wheeler trailers or simply come over to visit and just stay.
If you want to have an immigration discussion have an immigration discussion but still least start the discussion with your feet standing in reality.