They say he's racist for wanting to keep dark-skinned people from south of the border from coming to the United States.
They then say a wall won't work, anyway, and besides, more effective security would be more agents and technology. They favor these ideas because they say they are in favor of having strong border security.
But if we do have strong border security - something liberals/Democrats say they want - it will keep dark-skinned people from south of the border from coming to the United States.