The US was uneasy in annexing Texas because it would bring about an unbalance of power in the Senate between the North and the South who each wanted an equal number of representatives to side on with the issue of slavery.
When Sam Houston beat Antonio Lopez de Santa Anna at San Jacinto he immediately applied for statehood but President Jackson feared the revival of the slavery issue. President Van Buren followed suit with Jackson and did not approve the annexation of Texas.
The question of Texas, California and the New Mexican territory were increasingly prominent, and the sectional tension which they produced was destined to lead to the Civil War.
President Tyler (a previous Whig whose political party had abandoned him) by means of a joint resolution which only required a simple majority was successful in getting Texas admitted to the Union in 1845.