I'd say it was around 2014/2015, when there was a huge inordinate amount of focus on police brutality, and when "wokeism" started to become the prevailing cultural ideology on the left, promoted by none other than Obama himself. He relentlessly promoted the idea that women were earning 70 cents on the dollar of every man (not true when controlled) and that black men are being discriminated against by police (true, but divisive). Remember that Obama also promoted the TPP that even most of his party was opposed to, and Hillary was waffly on. That probably had an effect on this demographic that was unfavorable toward international trade deals. People forget but Obama's approval throughout most of his 2nd term was pretty awful, only in 2016 did it imrpove, probably because most of the political discussion was off of him and on Trump and Hillary. A lot of working class areas were being won by Republicans in 2014, whether it was Elise Stefanik's northern NY district, ME's 2nd district with Poliqun, or Iowa's 1st district. That was a sign, even as Republicans still did way better in the college-educated suburban districts in 2014. But 2016 really was a dramatic shift in the political coalitions that not a lot of people could predict the extent of.
All this is true but even so I don't think that they would have broken from the Democrats for just any Republican. A hardcore social conservative like Ted Cruz would IMO have still been even more of a turnoff for this demographic then wokeism. It took Trump himself combined with Hillary's ineptitude to bring the shift about.
