They argued it would lead to the abolition of slavery. Slaveholders feared that if slavery was banned in the territories, it would eventually be abolished in the states as well. They believed the Wilmot Proviso was the first step in a broader plan to undermine the institution of slavery in the United States.
They claimed it would upset the balance of power in the Senate. At the time, the number of slave states and free states in the Senate was evenly divided. Slaveholders worried that if slavery were banned in the territories, new free states would be admitted to the Union, upset the balance of power in the Senate and threaten their institution.