Why did most Northerners tend to oppose the Mexican-American War while most Southerners tended to support it?

The simple answer here is slavery.  The Northerners
thought that this was a war to expand slavery and the South liked the war because they
thought it would give them a chance to expand slavery.

I
should say that it's not that the North really wanted to end slavery.  They just didn't
want it to spread and they didn't want the South to get more political power.  They
thought that taking a bunch of land from Mexico would let slavery expand and would make
a bunch of new states that would have slaves and, thereby, would give the South more
political power.