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

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...

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.