Labor has long been a powerful Democratic constituency, and unions’ endorsements — not to mention their money — can be highly influential, especially in primaries. And as Democrats try to win back some of the working-class voters who supported President Trump in 2016, many of them are presenting themselves as left-wing populist alternatives to Mr. Trump’s right-wing populism.

With the Iowa caucuses more than seven months away, few of the big unions have settled on a candidate. Former Vice President Joseph R. Biden Jr. has received the only major labor endorsement so far, from the International Association of Fire Fighters.

A $15 minimum wage, described as a radical proposal just a few years ago, has largely become a point of consensus among the Democratic candidates. Several cities and states are phasing it in; in some places, including New York City, it is already in effect.

But the federal minimum remains $7.25, and while McDonald’s has stopped lobbying against legislation to increase it, it has not raised the bar for its own employees. The company argues that because most of its locations are franchises, it can’t mandate a higher minimum.

McDonald’s is also under fire for its handling of sexual harassment. Just this week, the Time’s Up Legal Defense Fund announced more than 20 complaints against the company, alleging not only sexual harassment but also gender-based discrimination, as well as retaliation when workers reported misconduct.