。
The United States is a representative democracy.
Literal
America [topic-は] representative democratic nation is.
代表的な adds 'representative/typical' to 民主国家 — the U.S. is being framed as a model democratic nation rather than just one example. 代表的 uses the productive ~的 suffix that turns Sino-Japanese nouns into descriptive adjectives. The formal copula である suits the textbook-style declarative register.