Bartels' methodology is deeply questionable. His definition of the "white working class" is whites in the bottom third of the income distribution, which comes to a bit more than one fourth of whites.
When people talk about the decline of "working-class" support for Democrats, they usually don't think of the working class as some exceptionally hard-bitten minority of poor people, they're thinking of that broad majority of whites who don't have advanced education or highly-paid professional jobs or own businesses or have large sums of cash in the bank. David Brooks' factoid (critiqued by Bartels) -- that in 2004 Bush won whites without a college degree by 23 points -- seems to be a better metric than Bartels'.
While it's interesting that voting patterns have diverged so sharply between working-class whites in the middle of the distribution and those at the very bottom, it's more interesting that those in the middle have shifted so heavily against the Dems.
SA