I can't find the article, but a Norwegian paper gave a link to a survey that showed 80+% of Trump supporters believe the election was sufficiently rigged that it affected the outcome

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Don't know how many people that is, though, to be fair. Some of the Trump voters may no longer identify as Trump supporters after his defeat. Supporters of strongmen tend to not like them anymore when they are no longer successful.
This is what I try to tell Trump supporters in various comments threads when they grasp at straws about various small incidents.