Honestly, it wouldn't take too many PTers striking at some of these hubs to absolutely tank production.
A fair number of hubs already depend on management to work during the summer, since they exhaust the double-shift list every single day. Happens earlier and earlier every year. That strike cushion isn't what it once was. They've already done it this week on one of our sorts.
It's early May. Not even hot out yet. The staffing only gets worse from here.
If the union agitates correctly, they could do a lot to increase PTer buy in to a strike. Most of the backbone (un)loaders in my hub are 2-to-8 years seniority by this point, and as a group are pretty agitated about the fact that the new retention bonuses ($120 a week) elevate new hires above their pay level. Extra insult comes from how production took a noticeable hit when management turnover started to dictate staffing training sups off the street. The resulting newbies have sucked.