For me it was the "I don't need any more votes" quote a few days before the election. The way he matter-of-factly stated that as if it was already set in stone spoke volumes...
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It sounds compelling but like the article states at the end, I'm not sure if it's going to change much. The DNC are way too timid to even consider challenging on something like this and the 2020 shenanigans have soured a lot of people on the "election fraud" claims.
If you put your tinfoil hat on you could even say that this was part of their tactic: make false claims to annoy everyone only to then commit fraud yourself later. It would he an almost perfect plan.
I don't think that's all that tin foil hat-y tbh. I think this is a classic case of "every accusation is a confession" from conservatives. Trump accused dems of stealing the election while literally trying to bribe governors for votes, and there's no reason to believe this election is different