As it relates to what I just posted, I don't dispute this except insofar as what exactly it means for Congress to "invoke" the 20th Amendment. I don't see the 20th Amendment as a mechanism for Congress to simply decide arbitrarily that a candidate "fails to qualify" for President.
I do.
In it's plainest meaning, it would mean that it was discovered that a candidate was younger than 35, not a resident for 14 years, or not a natural born citizen, since those are the only qualifications cited in the Article II.
US Code Title 3 lays out the January 6 procedure. If state legislature make an appeal to Congress that their Electoral College results are tainted by fraud, then it behooves Congress to listen to them.
Title 3 has language about how to deal with conflicting Electoral College votes vs. the intent of the state legislatures, so "failure to qualify" (in this case) would mean that enough states have declared their results to be invalid that the January 6 result is uncertain.
One interpretation of this may be that the 12th amendment says that the President is the person who receives a majority of the votes cast, not a majority of the possible votes cast. In this case, the invalid Electoral College votes are simply discarded and the candidate who wins the majority of the remaining votes becomes President.
The alternative interpretation (which I do not support) is that the 270 threshold is absolute, the discarded Electoral College votes don't lower the threshold, no candidate reaches 270, and the House of Representatives decides.
The reason that I discard the latter scenario is that at the time of the 12th amendment, two-party elections had not yet become the norm. Having the Electoral College vote split between three or four people was much more likely then than now, meaning that nobody reaching the 50%+1 threshold was more possible than now. In today's world, finding us in a 269-269 tie is so rare as to make tossing the election to the House almost unthinkable.
That's why I think the 20th amendment is relevant now. It is a final backstop against a fraudulent election where the results are rigged. A modern definition of "failure to qualify" would be when the President-elect was incorrectly selected by the Electoral College due to massive fraud committed by the Electoral College or by the processes that selected the Electoral College.
Just as how the states have plenary power to decide the method of choosing Electors, Congress has plenary power to declare what "failure to qualify" means. And what comes with plenary power is the wisdom and consent of the governed to exercise it. If the fear is of a tyrannical Congress that declares all opposite party candidates to have failed to qualify, the people would not support that abuse of power. But if the states methodically prove that a coordinated, systemic plot to undermine election integrity resulted in the wrong candidate winning, I believe the people would support Congress using whatever constitutional tools are at its disposal to correct the situation.
-PJ