Why did northern workers oppose the abolition of slavery?

They feared competition from black workers.

They believed slave labor was needed in the South.

They believed slave labor was superior to wage labor.

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They feared the South would develop better industries.

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The correct answer is the first, They feared competition from black workers.

Explanation:

The reason why some northern workers opposed abolitionism was because they saw the liberation of black people as a potential threat to their jobs because of competition.

It must be said though that records show it was a minority of norther workers who were explicitly and actively against abolitionism. Most of the time workers weren't active supporters because they saw themselves as wage slaves struggling to survive who didn't have time to fight for anything else.