You bet there's more up than just machine-related issues.
OK, let's recap the various ways things can go wrong:
1) "Fake voters". No fraud within the county election process; people set themselves up to vote in to counties or even two states, or vote when they're not able (no citizenship, felony, etc). Thing is, it's legally risky and takes a huge number of individuals to pull off. About the only time you COULD swing things is in a situation like New Hampster: small slightly conservative state next to a big liberal one. Trust me, at some point we'll deal with that: we'll get computerized lists of voter reg data from New Hampster and Mass, and cross-ref. No rush on that. Lemme know which New Hampster counties to focus on for that.
2) Old fashioned paper fraud. As one example, Napa County California did a dirty in the primaries: took a local race and in cases where voters didn't pick either, somebody in the elections office "marked the correct one for them". At least 38 cases of this were detected with forensic ink analysis.
When done right, electronic voting can actually protect against this. The Open Voting Consortium proposal for an open-source system involves an electronic station that prints a paper ballot which is readable in English plus has a bar code scan ability. You can take the paper to a bar code reader station and have the bar code read right there if you want. With the voter's intent recorded in three different places (plain text on the ballot, bar code on the ballot and serial number linked to an electronic record of the vote) tampering becomes damned difficult. See also
www.openvotingconsortium.org - I'm not connected, just a fan.
3) Electronic tampering at the precinct level. Basically, rig the machine to do it's totals wrong or substitute a certain percentage. Not easy unless you're the manufacturer. In Diebold's case, we know that literally megabytes of custom code at the touchscreen terminals was installed without any certification at all, illegally.
4) Rig the central tabulator. Most electronic systems use one central PC to tally the vote; security on most is suspect, documented as downright hideous in Diebold's case.
5) "Disenfranchise via scrubbing the registry rolls". SOME of this was done in Florida just before 2000...but most was caught and fixed beforehand. Basically, people whose names were similar to a convicted felon were scrubbed.
6) There are allegations that in Ohio, predominantly minority precincts got fewer per-capita voting machines which led to long lines in the rain and people going home without voting. I don't know if these allegations are true - IF they are, it's disturbing stuff. The credibility of all that needs to be looked at. BBV.org is aware of the complaints but is assigning this a lower priority than doing checks that involve potential machine tampering.
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What's supposed to PREVENT all this is public oversight. The entire process is supposed to be open to public and press scrutiny.
Which is one reason proprietary "God knows what it really does" software is such a problem. It's also a problem when the public and press are barred from the tabulator rooms, as is happening in Volusia County FL and one entire elections building in Ohio under the guise of "homeland security precautions"
. That sort of crap cannot be tolerated.