What is the right option? Feel free to elaborate.
If bernie Sanders hadn't dropped out I would have been voting/vote/have voted for him in the upcoming elections.
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This would be, strictly speaking, incorrect, as “upcoming” signifies the future and the above phrasing is past tense.
It may, however, be said (perhaps somewhat colloquially) as a variation of:
have been voting
This implies that your intention of voting for him was in the past, at some point in time. Perhaps something else has occurred that changed your mind. It may still be said as present tense, but isn’t as specific as:
This implies that, right now, in the present, you would vote for him.