Question

Why did some in the northwest and some antislavery forces call the agreement a "base betrayal by the South"

Answer 1

James Knox Polk, also called "Young Hickory" won the presidential election of 1844 with the combination of southern and western expansionism ideas. Dealing with a war with Mexico about the annexation of Texas and having to settle a treaty with the British empire over Oregon, they rapidly accepted what the UK had to offer. They submitter a drafty treaty extended the border between US and Canada along the 49th parallel. The treaty was ratified 3 days after.

As a result, a group of northwester expansionists, unsatisfied with the treaty, saying they cared more about annexing entire Texas but not entire Oregon, joined antislavery forces and condemned what they regarded a base betrayal by the South.