Prop 19 has seen an 8-point slide AGAINST legalization in the last week (44 for, 45 against, last I checked).
Hmm, nobody really posts against here anyway. Queue Mr. Hand...
It's entirely Constitutional for California to try to legalize, however, it's doomed to failure due to the Supremacy Clause and the gross over-reach of the Commerce Clause.
Now, California can still legalize, but at the Fed level, it will still be illegal. You can still be drug tested for work, fired for testing positive. That will be the street impact (no federal protections).
On the crime side, the Feds can go after whomever they want. THey will go after large dealers and distributors, and they'll be perfectly within their rights to put people in Federal slammers.
The Feds won't have cooperation from local law enforcement (wherever pot is legal), and that's the number one resource feds use to fight the drug war, so in that sense, the Feds will have incredibly poor vision for WHOM to go after in California. Thus, they'll only go after entities who are large enough to give them some publicity/reputation as heavies. As they have done to the moron in Colorado who said on the news openly that he was making 6 figures from his pot-growing operation.
Legalization in California will not help Mexico for at least a decade or two. As long as their is an illegal market, Mexican cartels will still own the distribution network. You won't see 'legal' pot in California start making it to the black market in Tennessee or Florida. The footprint of such a smuggling operation (as such it would be) would attract Fed attention. "Legal pot" will have a DNA footprint that's highly traceable.
One of the issues affecting the retrograde support in California is Gov. Arnold's efforts to decriminalize (which is not the same as legalization; the difference is that decriminalized pot gets you a fine, whereas legal means you don't have to worry bout nuthin). Decriminalization has a lot of the same arguments as legalization (we won't fill up the jails/ruin people's lives over a joint/we can generate government revenue off it in the same fashion as speeding tickets).
Decriminalization appeals to more moderates who don't smoke/don't care.
I predict California's prop 19 will not pass, by the way.