Richard, with respect, that’s a stretch. If you look at the numbers you cite there was no (overall) shift between 1996 and 2004, a time when the Republican stance on immigration generally toughened (if not by nearly enough). What happened after 2004 is that Democrats embarked on a winning streak. It’s no surprise, therefore, that businesses started to push more money in their direction.
This is not to deny that the rejection of Bush’s ill-judged, politically unwise, and generally idiotic immigration “reform” also played some part (businesses always like to push down labor costs as hard as they can) in this process. Had that nonsense actually passed, however, the resulting electoral cost to the GOP would have been far more devastating than any fall-off in contributions from, say, the “poultry & eggs” industry.