Every four years, the AFL-CIO labor federation engages in a quixotic mission: to attempt to dissuade many of its own members from voting for their first choice for president, the Republican candidate. Between one-quarter and one-third pull the lever for the GOP, anyway.

The labor leaders’ efforts have taken an added urgency this year because the Republican nominee, Donald Trump, may have the strongest appeal in decades of any GOP candidate to the union rank-and-file.