The Short Version
The claim is accurate. The V-Dem Institute's 2026 Democracy Report documents a 24% one-year drop in the U.S. Liberal Democracy Index score and a rank fall from 20th to 51st place. The Century Foundation's Democracy Meter also recorded a significant decline. While other indices like Freedom House and International IDEA did not report a downgrade, the claim only states the U.S. was downgraded in "a" democracy index — which is clearly supported by multiple credible sources.