Answer :
The policy opposed by both the free soil party and the republican party in the mid-nineteenth century was: Extension of slavery into the territories
In the mid-nineteenth century, both the republican and free soil party was known for their progressive views on Slavery. Both parties publicly denounce it and advocate their slavery.
Together, they tend to clash against the Democratic party which strongly supports slavery. At that time, slavery was predominantly in southern states. The Democratic party massively advocate it to be spread to northern states.
hearing this, both the Republican and free soil party massively rally their supporter to oppose it. They worked really hard for the legislation that prevent the extension of slavery to other territories.
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