The main reason why Northerners opposed the annexation of Texas was the issue of slavery. By the 1840s, the abolitionist movement had gained significant momentum in the North, and many Northerners were opposed to the expansion of slavery into new territories. They feared that annexing Texas would increase the political power of the pro-slavery South and make it more difficult to eventually abolish slavery.
Political Balance:
Northerners were concerned that annexing Texas would upset the delicate balance of power between free and slave states in the Senate. At the time, there were 15 free states and 15 slave states, and Northerners feared that adding Texas as a slave state would give the South a permanent advantage in the Senate and make it more difficult for the North to pass legislation that opposed slavery.
Economic Competition:
Some Northerners also opposed the annexation of Texas because they believed it would compete with the North economically. Texas was a large territory with abundant natural resources, and many Northerners feared that it would become a major producer of cotton and other agricultural products, which could hurt the economy of the North.
Manifest Destiny:
Northerners were not entirely opposed to the idea of territorial expansion. However, they believed that the United States should only acquire new territory through peaceful means and with the consent of the people living there. Many Northerners were uncomfortable with the idea of annexing Texas, which was still technically part of Mexico, without Mexico's consent.