At their level, I don't think his being Mexican has much to do with it. And I don't think Republicans hate him. He just did a poor job before the Judiciary Committee, (and maybe he's taking a fall for one of his bosses).
The anti-Mexican bigotry that exists in our country (like the anti-black bigotry of yesterday and today) is strongest amongst the less well-to-do and the less-educated--and the targets of their hate are typically someone lower on the totem pole. If you look at the old South, it wasn't the plantation owner class that disliked blacks so much as it was the poor white farmers. Then, as today, there are many who exploit this, and whip up fear and hatred. If Lou Dobbs et al can get working class people to be angry at other working class people, then they won't complain as much about their working conditions or status in society (while Dobbs and co. take it to the bank).