What's pushing him is, 1) he has Hispanics in his family, 2) he speaks Spanish, and 3) he's a friend of Vincente Fox.
That doesn’t explain the pro-illegal lobby, which includes pro-business libertarian think tanks and the Wall Street Journal. For them it boils down to the peculiar interest of cheap labor. Bush seems to be following their game plan, Willing Workers: ”Fixing” the Problem of Illegal Mexican Migration to the United States , a paper published by the Cato Institute. Bush is carrying water for Republian business interests. Add Rovian Hispanic pandering, and you’ve got your answer.