The Security Mentor

From Jon Stokes at the respected Ars Technica site:
What if I told you that it would take only one person—one highly motivated, but only moderately skilled bad apple, with either authorized or unauthorized access to the right company's internal computer network—to steal a statewide election?
He goes on:

In all this time, I've yet to find a good way to convey to the non-technical public how well and truly screwed up we presently are, six years after the Florida recount. So now it's time to hit the panic button: In this article, I'm going to show you how to steal an election.

He starts slow, with an overview of what kinds of voting machines are out in the field. Then it gets chilling. He explains all the places crooks could undetectably change election results and links to examples where security researchers have proven the attacks are possible.
The article is readable by anyone, not just security specialists. If you're in a hurry, the single most important sentence is

The only real protection against wholesale election fraud is genuine auditability, and that's a feature that paperless DREs[Direct Recording Equipment, the kind that count your vote as well as help you cast it] lack by design.

One way to look at the problem is to use what you already know. The voting machines are computers. In fact, they're standard PCs running standard Windows software. How reliable and secure do you think they will be?
Here's an even simpler explanation from user truesaer on Slashdot, the nerd message board. All you need to know is that the person sitting at the central counting machine can edit the results.