This earlier 2020 article is about COVID but it also applies to what-ifs related to Michael Flynn's recent statement.
The answer is that there isn't a lot of precedent in it but it seems extremely likely that Congress would immediately fight it. It ultimately comes down to who would follow illegal orders and who wouldn't. The state of Hawaii was under Martial Law for several years (which included things like Order 9066 that is universally condemned) but there is over 75 years of lawmaking between then and now. Even with a conservative Supreme Court it seems very unlikely they will be favorable to an executive's attempt to redo an election that their own branch has said was legitimate.