Unfortunately, it's a well-known psychological phenomenon that many (most?) people will start to believe a statement they hear repeatedly.

Before the 2020 election Trump primed his base by claiming over and over again that he could only lose if the election was rigged. Then, immediately after the 2020 election results were tallied, he and every right-wing media outlet spent the bulk of their airtime announcing that the election was stolen. It comes as no surprise that a significant fraction of the Republican voter base started to believe it was true, despite multiple investigations and dozens of court challenges about fraudulent votes failing to produce any evidence.

It's also not surprising that a significant fraction of the Republican base believes election fraud in general is rampant, which conveniently allowed Republican legislatures to make sweeping changes to how future elections would be held... like their standard voter suppression fare of closing polling locations, removing large blocks of voter registrations without notice, slowing processing of mail-in ballots and changing the rules regarding postmarks and late ballots, adding ID requirements, and, in some cases, revoking the validity of existing IDs.

All while the reality is that only around 1:10^6 votes are ever substantiated as fraudulent (e.g., Arizona has proven 36 cases of voter fraud among 42 million votes cast over the last 25 years).

See: https://www.brookings.edu/articles/how-widespread-is-electio...

Remember, Trump was mentored by Roy Cohn, who had been Joseph McCarthy's chief counsel in the 1950's. And McCarthy was famous for waving around "lists of known Communists" in the government and never actually showing them to anyone who could prove or disprove the claim.