Did Romney Win the White Urban Vote (answer: yes)

So I was thinking about the really interesting question of whether Obama's deficit among white voters (40-60) was a result of the rural-urban gap.  I haven't seen any exit polling that breaks things down that far, so I've resorted to cobbling together an estimate from a mix of exit polling and census data. 

If I were to spend more time on this, I could incorporate assumptions about voter turnout and the like.  But my basic conclusion is that urban whites are probably slightly Romney-leaning and are, at best, split 50-50.  The reasoning is fairly straightforward: urban minorities went for Obama around 80-20, and supply most of the margin of victory.  I end up with an estimate of around a 20% margin for Romney in urban areas, which is probably around 5-10% when you take into account the various assumptions I make that would overestimate the % minority vote. 

In order to do this I have to make a few assumptions, which means I'm certainly not 100% accurate.  I assume that I can use national census data to estimate the % of each minority that voted for obama.  Since minority turnout was slightly lower than white turnout, and this may have been even more the case in non-competitive states, I am probably overestimating by a few % the pro-Romney margin.  I also assume that african-american, latino, and other minorities had the same pro-Obama gap in both rural and urban areas.  If I had assumed that urban minorities were more pro-obama, this would mean a larger Romney margin among whites.  Lastly, I assume that the census and CNN have roughly similar definitions of "urban," but I suspect that CNN classifies far more people as "rural" than the census does.

I estimate that Romney's margin of victory among whites was: 32% in rural areas, 25% in suburban areas, 20% in urban areas.  Since Romney only won whites by 20%, it's clear that the differences between the census and exit poll data are shifting the results towards Romney.  This is looking at the national level though--it seems likely that Obama did better among whites in Ohio, for example, since he won that low-minority state. 

How do I get these number?  To use urban voters for example:
19% AA * 87% margin of victory = 17% of Obama's margin comes from African American voters.  23%*44%=10% margin from Latino voters.  Other minorities->3.5% margin.  Basically, Obama probably won urban minorities by around 80-20, and they accounted for 30% of his margin of victory.  However, Obama only won cities by 20%.  Given that white voters brought Obama's margin down by 10% and are about 50% of city-dwellers, that means about a 20% pro-Romney margin.

One other major source of "error" could be if the exit polls used a different definition of urban.  This, like lower minority turnout, would mean that I overestimate the percentage of minority voters and therefore assume a larger Romney margin among whites.  There is no way to check this directly, but let's assume that I at least got the minority margin of victory (80-20, or 60% margin) right.  And let's say I really think that Obama and Romney tied among whites in urban areas.  In that case, whites would compose 66% of urban voters.