A lexical verb has a dictionary meaning and can be marked for tense and 3rd person.
A modal cannot be marked for tense or 3rd person. A modal carries meaning and mood (speaker's opinion).
This is a humorous quip from Bill Clinton who was paraphrasing Winston Churchill on the Colbert show. (He did use correct grammar.) The original quote: "The best argument against democracy is a five-minute conversation with the average voter."