I was replying in the context of frgtpsswrdlame's post stating that employers had an incentive to pay nonunion members the same as union members, at least in th

I was replying in the context of frgtpsswrdlame's post stating that employers had an incentive to pay nonunion members the same as union members, at least in the short term. (pc86 responded to that post asking "If your dues are reasonable why would anyone leave?", and I responded to them.) In reality, there's no guarantee an employer would do this, and in theory the union could even negotiate to make sure it didn't – but it would have a pretty hard time justifying to its members why it was trying to keep other people's wages down. And in lieu of such negotiations, the larger the fraction of union employees, the less the employer has to lose by bumping nonunion employees' wages to match theirs, with the goal of ultimately weakening the union.