My impression is that although the Constitution requires American presidents to be NBCs, there is no formal mechanism in place to vet the candidates. In the past, I guess, candidates simply did not run who were not NBCs.
Expecting that kind of self-responsibility from Marxists is no different than expecting the immoral and amoral to have any respect for truth.
Actually, there are several mechanisms.
1. The DNC or the RNC must vet their candidates. I am sure that Independents or other parties must do the same.
2. Each Secretary of State must vet each candidate independently, before putting them on the individual State Ballots.
3. The Vice-President, before declaring the winning candidate in the Senate, must count Electoral Votes, verify that there are NO objections in the Senate. and THEN declare a victor.
Three requirements, all of them done incorrectly, leading to the problem we have today.