Which machines, where?
The machines that I work with at our polling places are an optical card reader and a touch screen system. Most votes are recorded by the card reader. Its probably computerized, I'd guess there's a Z80 or similar in there somewhere, but as it lacks an external network connection its going to be difficult to hack. The results cartridge is a memory pack that's about the size of a pack of playing cards with what looks like a three row DB type connector on it.
The touch screen unit obviously started off life as a PC. An early 1990s PC. It runs what looks suspiciously like vxWorks. It doesn't have a network connection. It also has a removable memory pack, a PCMCIA card which if I had a laptop from that era I could probably read.
This kit is pretty old but the overall system is actually pretty bulletproof. Ultimately any interference with a vote tabulator will show up as a statistical blip which will attract the attention of the county voting staff who will then use the paper trail to figure out what happened. (The 'blip' is quite obvious -- I usually work a split precinct, one that deals with the 'L-Z' half of the voter rolls for this are. Our tabulated results match very closely those of the 'A-K' crew.)
When you talk about hacking voting machines you really need to be a bit more specific. You also need to hack the entire system -- statistics being what it is any attempt to game the system (gerrymandering or voter suppression excepted) will show up like a beacon.