Final answer:
The admission of Texas into the U.S. was primarily stalled due to concerns that it would enter as a proslavery state, upsetting the balance between free and slave states. The annexation also entailed assuming Texas's large debt, and it reignited the debate over slavery's expansion westward.
Step-by-step explanation:
The main problem that stalled the admission of Texas into the United States was that Texas would be a proslavery state. The expansion of slavery into Texas was significant and had grown rapidly, and while many Texans found ways to circumvent Mexican laws that were restrictive of slavery, the prospect of Texas joining the United States raised concerns that it would upset the balance between free and slave states. Moreover, the annexation of Texas was also contentious because it would mean the assumption of a large national debt incurred by the Lone Star Republic. Finally, the debate over the westward expansion of slavery, which had been a contentious issue since at least the Missouri Compromise, was further ignited by Texas due to its large size and the fact that it permitted slavery.