The Texas annexation was so controversial because adding Texas to the United States would inflame sectional tensions between the abolitionist North and the slave-holding South. The Whigs were generally opposed to expansion that included slavery, while the Democrats supported such growth.
Because the Whigs held a majority in the Senate during the first half of the 1840s, they were able to stop the annexation of Texas for a time. However, near the end of his presidency, President Tyler, who was also a Whig but who supported the expansion of slavery, made a push to bring Texas into the Union. With strong backing from Democrats and pro-slavery southern Whigs, Tyler got the Congress to pass the annexation with simple majorities and sent the order to Texas on the last day of his presidency, March 3, 1845. Texas formally joined the United States on Dec. 29, 1845.