Organisation for Security and Cooperation in Europe:
The ODIHR began monitoring US elections after the disputed vote in 2000 which was ultimately resolved by the supreme court. The size of the observer missions was increased in 2016, and the 2020 contingent will remain at that level.
In many US states there is no guarantee that international observers will be allowed in polling stations.
The tone of the 2020 ODIHR report reflects far more alarm than in 2016, and notes that only some of its recommendations for improvements in the US electoral system have been acted on.