Which was the largest and most important issue that continued to surface during the era of "manifest destiny"?

Slavery

Explanation:

Manifest Destiny was the belief that America had the right and duty to expand its territory into Indian and Mexican territories. The problem was that during that time, slavery was already an issue in America. Northern states were against slavery and pushed for an abolition. Southern states believed that slavery was important for the country's economy. The addition of new territory added one more problem.

Southern states wanted these new territories to become pro-slavery territories, as a way to gain political influence. The Northern States did not agree with that, more pro-slavery territories would break the balance in the Union and maybe slavery would never be abolished.

If slavery would extend into the new areas that were settled.