Option: They did not want another slave holding state admitted to the Union.
Explanation:
Most of the Northerners opposed to the annexation of Texas because the state practiced slavery and the nature of Texas society. The Northern was opposed to slavery as they saw it as a crime. The North politicians wanted to maintain an equal balance of states which would bring stability of power in the Senate. The Southern people were quite eager to accept Texas as a slave state. Texas became acceptable to the North after Iowa joined as a free state.
They were accused that they had built their fortunes by swindling investors and taxpayers, bribing officials, and cheating on their contracts and debts.