Comment by jandrewrogers

Challenging cast ballots is non-scalable because it requires a lot of formal judicial process separately for each ballot challenged. Realistically a State could go after less than a thousand.

Even if you know with high certainty that a million ballots are fraudulent it is effectively impossible to challenge them because the judicial capacity required to do so doesn't remotely exist. There are adversarial scenarios where someone dumps vast numbers of fraudulent ballots and those ballots cannot be contested.

State legislatures have historically been empowered to vote in place of the people if the vote becomes infeasible or unreliable. People voting in Federal elections is largely a 20th century invention; the old methods are still available.