Why did slavery become an even bigger national political issue in the late 1840s?

The late 1840s were a time of great territorial expansion in the United States. Inspired by the notion of Manifest Destiny, tens of thousands of settlers headed out West to make new lives for themselves away from the increasingly overpopulated cities of the East. As settlers moved into the new territories, they brought many of America's social and political problems with them.
The most significant of these was slavery. Most Americans, even some of those who owned slaves, accepted that it was a thoroughly loathsome institution. But as slavery was so deeply entrenched in the economic and social life of so many states, it was politically daunting to abolish it completely. The issue was given added urgency by the United States' victory in the war with Mexico, which led to the incorporation of the slave state Texas into the Union.
However, if slavery couldn't be abolished, its spread could at least be halted. But even this proved controversial to supporters of slavery. As far as they were concerned, the new territories had every right to have slaves if that's what they really wanted. Abolitionists, however, were worried that this would lead to even greater entrenchment of the slave system's power in the American political system.
The idea of popular sovereignty became increasingly prominent at this time. This simply meant that settlers in the new territories would get to decide whether they wanted to have slavery. To die-hard abolitionists, however, this was an unacceptable compromise, as it did nothing to prevent the spread of slavery, let alone bring about its abolition. As we can see, then, the United States' expansion in the late 1840s greatly exacerbated the problem of slavery by making it harder than ever to find a practical solution.