It wasn't polarization but record turnout by the black population. I assume that without Obama the black turnout falls dramatically. I put it 2% above non-college educated whites instead of 7% above as it was in 2012.
The other driver of turnout recently has been the youth vote 18-30. Millennials were a big part in Obama's coalition. Neither Clinton nor Trump have done well at appealing to this demographic, so I think they also drop dramatically in turnout.
Black turnout has been rising consistently since the 2000 election, the drop in 2012 was really the only exception (although some sources have it going up from 2008 to 2012). I believe proportionally the turnout stayed relatively the same in 2010 and 2014, which doesn't bode well for the "Obama-Centric" AA turnout theory.
The youth vote didn't really "spike" much in 2008, and certainly not in 2012...that's largely a myth. It's been fairly consistent with national turnout overall. 2004 had larger youth turnout than 2012.
Was going to post similar graphs, but you beat me to it!
However, I don't think those numbers are correct. Most other sources have black turnout rising higher in 2012 than 2008. I don't know why there are so many discrepancies about turnout rates among demographics but as I understand it, it went something like 64% - 65% (2008) ==> 66% - 67% (2012).
Either way, black turnout was already on the rise and it continued to stay elevated even during the midterms (relative towards traditional midterm turnout, of course). Youth vote was also not high-turnout as many believe in 2012. It could go lower, but if going by historical trends, not by much.
