In response, Donald Trump issued a press release.
"Sadly with this endorsement of Hillary Clinton - who is totally owned by Wall Street," it began, "the leadership of the AFL-CIO has made clear that it no longer represents American workers. Instead they have become part of the rigged system in Washington, D.C. that benefits only the insiders. I believe their members will be voting for me in much larger numbers than for her."
He went on to argue broadly against Clinton's candidacy from the perspective of a working person, concluding with the sort of hyperbole that we barely even notice any more: "I will fight harder for American workers than anyone ever has, and I will fight for their right to elect leaders who will do the same."
Support from working class white voters is integral to Trump's general election strategy. He has repeatedly pointed to Ronald Reagan's support from white Democrats in 1980 as a possible blueprint to victory -- and members of labor unions were an important part of Reagan's success.
Exit poll data shows how Reagan overperformed with members of union households in 1980 -- relative both to 1976 and to every presidential election since. This is union households, not union members, so this isn't a perfectly precise measure of union support. But it makes the point.
NBC News was generous enough to share data from this election cycle on how union members are leaning. In January, Clinton led Trump by 9 points with this group; last month, the lead was slightly narrower. That's much closer than we saw in 2012, but this is also with a substantial number of voters who didn't name either of the two front-runners (7 and 12 percent respectively).
That doesn't tell us a whole lot. But we know that Trump probably won't replicate Reagan's 1980 win on the back of union members for another reason: There are a lot fewer of them, and they're a lot less likely to be white.
Since 1984, the percentage of Americans who are members of unions has dropped from just under a fifth to just over a tenth. The percentage represented by a union -- people under union contract but who don't pay dues -- is slightly higher. But only slightly.
Data from the Bureau of Labor Statistics shows how the membership of those unions has shifted. That data only goes back to 2000, but even since then, there's been a big shift.
In 2000, 45 percent of union members were white men. In 2015, the most recent year for which we have data, the figure was 38 percent. In 2000, 73 percent of union members were white, with 60 percent of white union members being male. Last year, 69 percent were white -- but 45 percent were women. Given Trump's unpopularity with women and non-white voters, those demographics have shifted to Trump's disadvantage.
And that's just since 2000, much less 1980.
It seems quite possible, if not likely, that Trump will outperform Mitt Romney among union members. But, as with his appeal to non-college-educated white men (a group which with union membership often overlaps), it's likely not a big enough group to swing the election the way that Reagan dominated in 1980.
Or, for that matter, to win.