Actually it’s at the very core of constitutional to declare such a thing. When you have an extra-constitutional election and an extra-constitutional government as a result, the only means of restoring the constitution are also extra-constitutional, because the whole system is outside of the constitution.

That’s where the military and their oaths come in to play. They are sworn to uphold the constitution. When a constitutional crisis occurs, it is the militaries duty to restore the constitution. That is not a constitutional act but actually an extra-constitutional act, to my knowledge at least.

I’m not sure if any such thing has ever occurred, which is why no one really knows how it would play out. There are some that believe that trump would be declared the rightful winner of the election and therefore reinstated as president. There is a separate theory that continuity of government protocols would come into play, which would all come down to the military and be far outside of the political continuity of government, since the whole election was fraudulent, and ultimately the military would at some point hold an election and carry out the whole election on their own. From my own research, I believe that the second scenario is the most likely one to be true, but I am by no means certain as such continuity of government protocols are likely beyond classified.