[AndyJackson]

How far can they go ... without seeming to write off what was once regarded as a core progressive constituency?

Oh they wrote them off a long time ago, but Republicans also wrote them off. It's Trump who figured out how to make a campaign out of what was going on.

[Maine Mariner]

Reagan did not write them off, neither did Nixon.
In fact a good number of unions endorsed Nixon in 1972
and the AFL-CIO did not endorse either Nixon or McGovern.

President Nixon was on the verge of realigning the political map in 1972. The blue collar union members and yes even a good number of the union leaders had an intense dislike for McGovern and his crew. I am convinced that the real reason the MSM and the Left pushed and pushed on Watergate was to end that realignment.

[Ouderkirk]

It's Trump who figured out how to make a campaign out of what was going on.

And that, in a very few words, is the sum of Trump's campaign. His supporters have no-one else to turn to. Both parties have forsaken the working classes. The vulgar lumpenproletariat. They both detest them, and are working to reduce them to right-less serfs.