You may not have heard of this until now, but a doomsday is fast approaching that could halt a lot of UPS's operations across the country -- which would seriously screw up the supply chain ... among other facets of American life that rely on the shipping/receiving giant.

Upwards of 325,000 UPS employees are primed to stop working starting on August 1 if their Teamsters union and their UPS bosses can't strike a new labor contract ... which has been getting ironed out in recent months, but which has hit a snag on the money front.

Specifically, part-time workers' pay is at issue. At the moment, UPS says its PT staff -- which makes up a significant number of its workforce across the company -- makes about $20/hour on average, after completing a 30-day period. The union is asking for a serious raise.

The union's leader, Prez Sean O'Brien, has been quoted with some fiery language over the prospect of UPS workers hitting the streets. During a rally speech in Atlanta Saturday, per CNN Business, he said, "We’ve legalized, strategized, now it’s time to pulverize."

BTW, this would-be strike wouldn't just affect ground transportation -- a separate union repping UPS pilots has apparently also committed to joining in solidarity if an agreement can't be reached soon ... so this would be felt both domestically and abroad too.

Of course, the backdrop of all this is the SAG and WGA strikes that are going on right now -- whose members are demanding a number of things as well in their own negotiations.