"what percentage of votes"
A small percentage would come from the families of the illegals who are themselves legal. Also, Bush's willingness to help the illegals achieve legal status is being perceived as a true token of friendship between the republican party and latinos, in general. Also, once he's gotten his way in this, he can then "clamp down" on any future amnesty. This would be taken with a certain amount of understanding from the latino community, who realize that there's only a finite number of jobs to go around, that you just can't keep the gates open forever, etc. The Irish and Italians in the 20s felt the exact same way - glad to get here, work here, but didn't want a whole lot of competition flooding in after they got here.
By the way, just to make sure we're clear: I think anyone who wants to give illegals licenses to drive or PTA votes should be taken out and beaten. I think that's crazy. Normalizing their status is one thing (read: amnesty), but to allow an illegal to walk in, say "hi, I'm an illegal alien, can you please issue me a valid driver's license" is absolutely insain.