Actually, I think race riots are the last thing he wants. And that is why he jumped in. The best outcome for him is the Feds being able to throw Zimmerman in prison on some technicality. If Zimmerman goes scot-free (as he probably should, given the local assessment of his lack of culpability), the odds are non-zero that Rodney King style riots could ensue. And that would probably result in a GOP landslide in November, no matter who is nominated.
A Washington Insider on the Ulsterman Report stated last year that the Obama Administration was planning on using race to cause civil strife this summer during the presidential race.
Go see and read for yourself:
http://theulstermanreport.com/2012/03/24/insider-warned-us-obamas-racial-division-plan-now-underway/