Why did anti-slavery members of congress oppose admitting Texas to the union?

Anti-slavery members of Congress opposed admitting Texas to the Union primarily because it would extend the institution of slavery into new territories. Texas was a slave state, and its admission would disrupt the balance of power between free and slave states in Congress. Additionally, many abolitionists viewed the annexation of Texas as an expansion of slaveholding interests, which conflicted with their goal of ending slavery in the United States.