Finally I found links to some pieces that provide some evidence for this assertion. I have to admit that the main article discussing this is two years old. (Well, better late than never.) The author, Sean Trende, is a data-obsessed analyst, as demonstrated by him declining to 'unskew' his reading of poll data in 2012.
Trende reports that demographic projections show that 6.5 million white voters sat out the 2012 election unexpectedly. The map below shows lower turnout in blue and higher turnout in red:
Image: realclearpolitics.com
Trende found that areas with a strong Perot vote in 1992 showed declines from expected turnout. He describes a particular kind of voter who seemed to stay away:
[They were] secular, blue-collar, often rural voters who were turned off by Bill Clinton’s perceived liberalism and George H.W. Bush’s elitism. They were largely concentrated in the North and Mountain West...However, this 'Perot' effect was hardly the biggest factor. The Perot vote was a smaller factor than Hurricane Sandy in keeping people from the polls.
Despite an interesting analysis, Trende has perhaps identified only a small part of the reason for the missing white voters. Still, no one has shown why so many whites stayed home. So those claims about the candidate 'not being conservative enough' are quite empty, no matter how loudly made.
Image: foxnews.com
Extras. Poorly done appeals to blue-collar workers here. Site with easy to read statistics. The coal miners shown above and their incentive to attend the Romney rally or else.
Update 2/25/16. More evidence that True Conservatives didn't stay home in droves. Of course, you have to care about data and evidence to be swayed by this information. No True Conservative or True Believer would allow themselves to doubt the legend of the golden horde of lost conservatives.
