And no I did not mean secession. What Im positing is a war over a disputed succession to the presidency. With any pod after 1790, make an American War of Succession possible.
Allowing military personnel to also be government officials might help. If the losing candidate commands a large portion of the army, and they're more loyal to him than to the Constitution...
I suppose it depends on how you define the word "War". I suspect that if Horatio Seymour or Samuel Tilden managed to cross the threshold in 1868 and 1876 then Ulysses S. Grant leads the army to capture the White House and invalidate the results of the election. This would happen very quickly, and would be more of a coup, then war of succession.