They didn't elect Lincoln, and so far as he behaved, he was the closest thing to a Monarch we've ever had. I dare say no other President so abused his powers as this man, but Woodrow Wilson locking up some 35 thousand political prisoners comes close. I suppose Roosevelt does too.
But the form of government is irrelevant to the right of independence. A lot of dictators claim they are a Republic or a Democracy, but they are in fact dictatorships.