I would definitely expect open source to be much more tamper resistant than proprietary code that no one is allowed to audit.

I, of course, have to agree that to do a relevant audit you need the source. But were I one of the idiots that think it is fun to tamper I would find it much easier to tamper with something where I knew the source than something where I did not.

However much the source is known and audited, with a known source it will be easier to sneak 'additional records' in or even to modify records. If you want to tamper with the data you do not even need to modify the actual audited voting code all you need is to intercept records, the layout =etc of which you know from you access to the source, amd modify them in the stream.

Erik