The Hill has published an article with the premise that there's no reason to not expect minority participation to be at 2008 levels, especially since their population increased. There are reasons, of course, as voter enthusiasm was higher to elect the first minority President.
Setting that aside, the big argument isn't with minority voters. It's with White voters. In the two party vote, Barack Obama got 44.5% of them in 2008. That beat Al Gore's 43%, John Kerry's 41.5%, and far exceeded the 38% share for congressional Democrats in 2010. President Obama's job approval among White voters has been under 40% for some time. The problem with these polls is that they assume all 36 year old White males who make $50,000 are interchangeable. So they include White Democrats instead of White Republicans
Pew has Obama getting 46.2% of Whites in a pro-rated two party vote. Fox has him getting 43.4%. I think he's going to get in the 40% range, but it could be lower.