When Trump was facing impeachment there were threats from some of his supporters that they would plunge the country into a civil war. Supposing the election is close, what if he refuses to leave the White House after losing the upcoming election and his supporters take up arms to keep him in office, would the US government and new president cave into his demands by forming a coalition?
Note: I originally considered voting to close the question as too hypothetical, but technically I think it has a logical answer.
This scenario describes a coup: taking power by force or threat. The success of a coup usually depends whether the army supports it or not. If the coup succeeds the dictator has no reason to form a coalition with anybody. If the coup fails, the wannabe dictator and their accomplices are judged for treason, not invited to form a coalition.
 As suggested in the comments, in this case the Secret Service would probably just remove the non-elected president from the White House (unless they decide to support the coup, which is very unlikely). Presumably the illegal attempt to seize power would still be punishable.