So, we go back to the RATs in the back room of the union hall premarking hundreds of paper ballots, or prepunching stacks of butterfly ballot cards (Hanging and dimpled chads, remember?) or "finding" another box of votes in a car trunk once they learn how many they need, or losing boxes of votes from dependably conservative districts?
The machines in use in (our part of, at least) New Jersey are not connected to any internet or phone line. When the machine is turned on in the morning it prints out a "zero proof", showing the choices available and the votes for each, which should all be zero. At the close of voting it prints out the results. The total votes cast are matched up with the number of voters signed in by a voter authority slip for each.
The only way the machines could be "jiggered" would be in the warehouses in which they are stored between elections, each one individually. That would require a plethora of politically motivated or bribed technicians working full time in the off season. And SOMEONE would eventually expose it.
I don’t see how the firmware, chips and software could possibly be contentiously audited to ensure that no one has introduced code to cheat.
It would not take that many lines of code to subtlety swing an election.
Tampering could be done at time of manufacture, during maintenance, transport, setup, updating. Or, in certain states, tampering would only need to be in a handful of precincts and that would swing statewide results.
Certainly cheating is also easy to do with paper ballots, or mechanical machines, but better, tighter human control of the voting and counting process could mitigate those cheating schemes, with electronic code, not so much.