At the top of a slender ​98-foot-tall traffic camera tower in central Seoul, Kim Hyoung-su is living under a tarpaulin shelter. It is so small he can’t stretch fully when he sleeps. But Mr. Kim has been up there for 77 days, protesting one of the biggest economic problems in South Korea — labor inequality.

It’s an issue that is sharply dividing candidates campaigning for the presidential election​ next Tuesday.

“I feel like an animal in a cage, eating, sleeping and relieving myself in the same place,” Mr. Kim said from his midair protest site. “But I will persist if this is what it takes to let society know the reality workers like me face.”

Mr. Kim, 52, is one of thousands of subcontracted workers at the Hanwha Ocean shipyard, one of the largest in an industry that is a pillar of South Korea’s economy. Workers like him commute by the same bus, wear the same uniform, eat at the same factory mess hall and work on the same ship as those hired directly by Hanwha at the shipyard, located on the south coast. But they are paid only half of what the others earn, Mr. Kim said.

​Mr. Kim climbed the tower in front of the Hanwha headquarters in Seoul on March 14 to protest the “discrimination I couldn’t stand anymore.”