Final answer:
Members in Congress opposed the Mexican-American War due to concerns it would disrupt the national balance over slavery, as expansion posed the risk of new slave states. The war was seen by some, including Abraham Lincoln and Whigs, as a ploy to extend slavery's reach.
Step-by-step explanation:
Some members in Congress opposed the Mexican-American War because they feared it would reignite national controversies over slavery. Notably, figures such as John C. Calhoun and Abraham Lincoln were concerned that the acquisition of new territories from Mexico would lead to disputes over whether these areas should be free or slave states, potentially destabilizing the nation's delicate balance on the slavery issue.
Moreover, critics like the Whigs and abolitionists argued that President James Polk had instigated the war to expand slave territory, which they found morally and politically unacceptable. The debate intensified after the war with proposals like the Wilmot Proviso, which sought to ban slavery in any new territories, but it failed to pass. These discussions foreshadowed the major sectional crises that followed, culminating in the Civil War.