In what ways did the Mexican-American War impact national politics?

The Mexican-American War resulted from years of disputes between the United States and Mexico over territory in present day Texas, California, and New Mexico. These disputes arose in large part because of American ambitions surrounding western expansion and "Manifest Destiny," the belief that America's expansion across North America to the Pacific was ordained by God and necessary to civilize the continent, no matter who might already inhabit it. In fact, President James K. Polk won the 1844 election with his slogan "54-40 or fight," referring to his desire to take all of the Oregon Territory that was shared with Britain, and he proved himself a staunch advocate of expansion. In the 1830s, the Mexican government invited US citizens to settle in Texas as a way to encourage the development of the region. However, when Mexico outlawed slavery and attempted to impose Catholicism on settlers, Texans led by Stephen Austin declared independence and created the Republic of Texas. The Republic of Texas sought annexation by the US, but controversies over slavery and the possibility of war with Mexico caused most presidents to avoid the issue. However, when gold was discovered in California, President Polk decided that it was time to gain the territory held by Mexico. The resulting Mexican-American War lasted from 1846 to 1848 and resulted in the Treaty of Guadalupe-Hidalgo.
The treaty proved favorable to the United States, which received the territories of Texas, New Mexico, and California in exchange for just $15 million. However, this land quickly fueled debates in the United States. Slaveholding states feared that abolitionists would outlaw slavery in the new territories, tipping the balance between slave and non-slave states in Congress in favor of non-slave states, particularly northern states. On the other hand, northerners and abolitionists feared that the growing cotton industry would spread to these new territories, taking slavery with it. As the California Gold Rush intensified, the population increase in the territory made it eligible for statehood. The Missouri Compromise (1820) that had set a precedent for keeping the balance between free and slave states only applied to the Louisiana Territory, and new debates broke out over whether to extend the 36-30 compromise line to the Pacific, to outlaw the spread of slavery altogether, or to allow slavery in territories based on popular sovereignty (letting the people of the territory decide). Ultimately Congress agreed to the Compromise of 1850, which admitted California as a free state and allowed for popular sovereignty in the territories of Utah and New Mexico. It also outlawed the slave trade in Washington, DC, and included a much stricter Fugitive Slave Law that was abhorred by many northerners. Ultimately, although it ended in compromise, the debates surrounding the territories from the Treaty of Guadalupe-Hidalgo revealed the growing tensions between the North and South. The Compromise of 1850 succeeded in keeping a tenuous peace for just four years. Then, the issue of slavery in Kansas and Nebraska led to violent disputes between proslavery and antislavery factions.

First, there was the impact of the war itself:  many Northerners saw this as a Southern conspiracy to spread slavery.  The war was ordered by a Southern president (James K. Polk) and instigated by a Southern general (Zachary Taylor).  Taylor moved his forces into the disputed boundary between Mexico and the United States between the Nuecces and the Rio Grande Rivers.  Mexico fired on the American soldiers, and Polk declared war.  Abraham Lincoln, who was then a Whig, argued that American blood was not shed on American soil and that the United States instigated the conflict.  Of course, his pro-expansionist constituency in Illinois did not like this, and he lost in the next election.  
After the war, there was a question of whether or not the new land would be free or slave.  Southerners did not want to lose the split between free states and slave states in the Senate, and Northerners wanted to control the spread of slavery.  All of this would lead to the Compromise of 1850 which, ultimately, made no one happy.