abolitionists
The two primary groups opposed to the Mexican-American War were abolitionists and some Whigs. Abolitionists opposed the war because they feared it would expand slave territory and perpetuate slavery in new states. Meanwhile, many Whigs criticized the war on moral and constitutional grounds, arguing it was an unjust aggression against Mexico and a distraction from domestic issues.
Slaveholders opposed the Wilmot Proviso because it aimed to prohibit slavery in the territories acquired from Mexico after the Mexican-American War. They feared it would limit the expansion of slavery into new territories and potentially undermine the balance of power between free and slave states in Congress.
Because it was seen as favoring slavery.
Because the acquisition of new territories would undermine the delicate balance between free and slave territories. This in fact is what happened, eventually becoming the cause of the American Civil War (1861 - 1865)
they feared it would weaken the country's independence
NAFTA was opposed by the Teamsters and other labor unions. They feared that trucks entering the US from Mexico were unsafe.
Southerners opposed the Wilmot Proviso because it sought to ban slavery in territories acquired from Mexico, which threatened the balance of power between slave and free states in the U.S. They believed it went against their rights to bring slaves into new territories and feared it could lead to the restriction of slavery in existing states.
As the war was motivated by pro-slavery interests, northerners feared the acquired territory would be used by slavers (they did, especially on New Mexico and Texas). This eventually upset the balance of power between pro and anti slave interests and resulted in the American Civil War (1861-1865).
Various groups opposed the abolitionist movement, including Southern slaveholders, pro-slavery politicians, and white supremacist organizations like the Ku Klux Klan. These groups benefited economically and socially from the institution of slavery and feared the consequences of its abolition, such as loss of labor and power dynamics shifting. They often used violence, intimidation, and legal mechanisms to preserve slavery and suppress the abolitionist cause.
immigration
Completely Destroy Them then especially the African Americans feared that slavery would still continue
They feared the loss of jobs and damage to America's traditions.