which concern about Texas joining the union was the most significant in the debate that took place within the united states in the 1840s?

Slavery and relationship with Mexico

Explanation:

Firstly the United States was worry about Slavery, they thought if joined another slave state could have political instability in the country. Texas joined 28° state of the United States and like a slave state.

The other concern was the relationship's Mexico, but the North American country Analyzed advantages and disadvantages, there were more benefits to get Texas, the United States tried to buy that land but Mexico refuses the offer and then Mexico was invaded, both countries broke relationships.