Torie you need to read your article.
He uses a mid term election during a tough economic time as his worst case scenario. Parties in power lose midterms all the time. It is the norm. I would not use it to extrapolate presidential elections.
And I don't think anyone on this forum or beyond thinks the Democratic party can jettison white working class voters. That's absurd. Why do you think Dems are so chummy with unions? Only getting 40% of that vote is an electoral drubbing, but even your article states 40% almost ensures a perpetual lock on the White House.
That still shows an issue with working class whites. What % of working class whites are in unions compared to earlier eras? The non-union working class white voter has a different set of concerns from the unionised one; a set of concerns that the Democrats may have trouble addressing.