James K. Polk was the 11th President of the USA, serving 1845-49. The lyrics race through the man’s accomplishments, and suggest that he was a successful president not needing to seek a second term. If so, then why is Polk not more on our minds, wonder They Might Be Giants?
It is not clear if they are being cheeky (or if we are too cynical to imagine a neutral biography of a politician). Polk basically governed over a time when the map of the entire continental United States was set, though ethically today we might question territorial expansion (Texas) and invading Mexico. This was part of “manifest destiny,” a belief in the superiority of Americans necessitating expansion.