Yesterday, in the course of my postmortem on the rather resounding defeat of the campaign to unionize the workers at Amazon’s warehouse in Bessemer, Alabama, I noted that the change in the class composition of America’s unionized workforce was accelerating. More than ever, perhaps, the dividing line between workers who can win a union and workers who can’t depends on their replaceability, clunky though that word may sound.

In my article yesterday, I pointed out that blue-collar and low-paid service sector workers fear being discharged, harassed, or downgraded by management, or even having their workplace closed, for supporting unionization, even though many such management threats violate federal labor law. (The penalties for such violations, however, are altogether negligible.) Professionals, by contrast, usually know that management would have trouble finding and training their replacements, and that they have a decent chance of finding comparable employment even if they’re let go. (That’s why unions of professional athletes wield such clout.)

It’s not surprising, then, that professionals have continued to unionize during the past few years, in media outlets, at nonprofits, in school districts and universities, and in hospitals. Nor is it surprising that successful unionization campaigns among blue-collar workers have been few and far between.

I’m returning to this topic today because of two press releases that popped up this morning in my in-box. One was from the NewsGuild of New York (an affiliate of the Communications Workers of America), announcing that a majority of the 650 tech workers at The New York Times had formed a union and were seeking voluntary recognition from management. (The 1,300 Times reporters, editors, photographers, and such are already unionized.) The tech workers who’ve decided to go union include engineers, product managers, project managers, designers, quality assurance staffers, and data scientists and analysts.

Also in my in-box was an announcement from the Nonprofit Professional Employees Union, a relatively new organization that has racked up a string of organizing victories. It informed me that employees at the Brookings Institution and the Urban Institute—two anchors of the think-tank world—had each formed a staff union and, like their fellow workers at the Times, were calling on management to voluntarily recognize them. In a wonderful display of entirely justifiable chutzpah, they cited their respective institutions’ many studies attesting to the value of unions as one reason for their decision to unionize.

Our organizations have produced independent research that shows the value of unions in reducing racial and gendered wage disparities, increasing economic mobility, and strengthening workers’ voices. It is our belief in this research—and our commitment to each other—that inspired us to form our respective unions.

Being a professional is not the sole determinant of workers’ ability to win a unionized workplace. The nature of one’s employer is the other crucial factor. The engineers and mathematicians at Amazon and Microsoft have no unions because the pay is good and management won’t stand for it—though if 80 percent of those workers truly wanted a union, management would certainly have to rethink.

But for working-class employees of for-profit businesses—that is, for the very workers who most need the raises and benefits that unionized status confers—unionization remains out of reach, and will stay that way unless Congress rewrites labor law to keep management from intimidating workers who’d like to join a union—as the PRO Act, currently pending in the Senate, would begin to do.