Most of Conscience Whigs believed the United States had no rights to anything South of the Nueces River. They opposed to the Mexican War because they were afraid that the extension of slavery to new territories would jeopardize the republic.
Transcendentalists were against the war because of the immoral, corrupt and egoistic principals behind it.
Southern expansionists, including President Polk, supported the Mexican American War at all costs. Their main goal was to conquer more territory from Mexico so they would have more slave states.