The election apparatus in the US is extremely hard to rig. It’s run by local officials, so in order to fix the counting of voters you need to get to thousands of individual county/city/town election boards, all at once. Those boards have members of all parties generally present on them so there is a fair amount of local oversight to overcome, too.

There was a bit of time in the early 2000’s where the voting machines themselves were suspect but some good work by independant researchers shined some daylight on that. Now most votes in the US are either done purely by paper ballots (counted by machines) or on machines that generate verifiable paper trails, and are very hard to just casually alter the count without being found out.

Republicans rig the vote by manipulating their media. Roger Ailes was one of Nixon’s media advisors during Watergate. The lesson he took away was that if the media didn’t hold Nixon to account, he would have never had to resign. Ailes went on to run Fox News in the mid-90s, and the rest is history.