Final answer:
The word 'swelling' in Douglass's phrase 'swelling vanity' is used to describe America's vanity as excessive and overblown, emphasizing its negative connotations.
Step-by-step explanation:
The effect of the word swelling in Douglass's phrase, swelling vanity, is to make America's vanity seem bloated, pompous, and filled with an overweening pride. This usage emphasizes the negative aspect of vanity, suggesting it has grown beyond reasonable proportions and is now something grotesque. Douglass uses this term to convey a strong criticism of America's self-perception, especially in the context of its national celebrations and the triumphant narratives that ignore the country's history of slavery and oppression.