There is a fallback if no one undisputed is set to be sworn in on Jan. 20. According to the 1947 law establis>hing the line of succession to the president, if the president and vice president are unable to be inaugurated, an "acting president" takes over until the situation is resolved. First in line would be the speaker of the House, and second would be the president pro tempore of the Senate, as long as the official resigned their seat in Congress. Beyond those two positions is a list of cabinet positions. https://cookpolitical.com/analysis/national/national-politics/six-scenarios-confusion-counting-2020-ballots-not-just-scary