Anyone who makes definitive claims that voter fraud doesn't exist, or is only a minor inconsequential issue, is either stating this from an agenda-driven perspective, or from ignorance. Democrat voter fraud goes back a long way, and was famous during the Richard J. Daley days in Chicago. Although fraud has not been definitively proven in the 1960 Presidential election, JFK won Illinois by 9000 votes, though Nixon won 92 of the 101 Illinois counties. Chicago, and Cook county, run by Daley and the democrats, gave JFK a 450,000 vote margin win in that county - even though the TOTAL population of Chicago at that time was ~3.55 million. And, Daley held back the Chicago vote until late the next morning.
The bottom line is that you don't have to change the popular vote much, just need to control votes in specific places. Yes, it is doable, and likely easier now than ever before - given the technology we use.
Daley held back the Chicago vote until late the next morning.
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Even if Illinois election results were fraudulent, it did not give the presidency to Kennedy. Take Illinois electoral votes from Kennedy and give them to Nixon and Kennedy still wins the presidency.