Final answer:
Southern political leaders praised the Dred Scott decision as it legally protected and expanded the institution of slavery making it a national institution, and was seen as a blow to northern abolitionist and the Republican Party.
Step-by-step explanation:
Most Southern political leaders praised the Supreme Court decision in Dred Scott v. Sanford (1857) because it provided legal justification for the protection and expansion of slavery throughout the United States. The decision essentially stated that Black people could not be citizens and that Congress had no authority to prohibit slavery in the territories, which solidified the practice of slavery as a national institution. This ruling was seen as a victory against northern abolitionist efforts and the Republican Party's platform, which opposed the expansion of slavery.