Edit: downvoted already, interesting. I lean centre-right on most things
1) You'd have to imprison millions of people to make a difference in most elections.
2) If you actually have the power to imprison so many people without consequences, you don't need to do it for the election's sake. Just fake the election results. Works for Putin, Lukashenko, and other dictators. Gulags are a pain to manage, especially in the modern world where information spreads like wildfire.
And how about what’s made illegal in the first place? Could there be an incentive to pass laws that criminalize activity that the other side is more likely to engage in? (See: the war on drugs)
It’s probably naive to think that there are no political consequences at all.
As one Watergate conspirator revealed in an interview, this was the purpose of the war on drugs: "by getting the public to associate the hippies with marijuana and blacks with heroin, and then criminalizing both heavily, we could disrupt those communities." [1]