Perhaps the knowledge that a second vote is coming up would lead to harsher negotiation over the terms of secession. It’s hard to estimate how likely that is, but one side or the other can always refuse a bad enough deal and default to the norms between nations, or even return to the question of the ballot or the bullet. Additionally, most secession movements today would result in a new country in close proximity, with bonds of trade, family and familiarity which create pressure towards a fair-ish deal.
What’s more, knowing that there’s another vote and a deeply disaffected minority who’ll be your angry voters if they remain will lead to more generous negotiation, in the spirit of “don’t let the door hit you on the way out.”
Perhaps a second vote could be anti-democratic: the will of the voters in voting “Leave” should be respected! This is a tremendously strong argument, given the perception that elites ignore popular opinion. But the argument cuts both ways. If the elites negotiate a deal other than the one that informed popular opinion, it is profoundly democratic to let the voters have their say.
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