Unless you're one of those ornery folks who believe that only politically engaged Americans should vote, there aren't many good reasons to oppose efforts to expand access to the ballot. Voter fraud is quite rare, and voting fraud — an organized effort to illegally disrupt elections — is hard to organize. So you might think that any restriction on the way someone can vote will unfairly marginalize potentially legitimate voters.
That's true, with one big exception: internet voting.
No doubt — nationwide internet voting has an intuitive appeal. It would decrease the costs of elections. It would dramatically increase turn-out. It would allow marginalized communities to avoid harassment at polling sites. It would speed the vote count. A majority of voters regularly endorse the idea.
There are two main reasons, though, why internet voting is, at best, a dream best realized 20 years in the future — if ever.
The internet is not secure. It does not matter whether results are sent to an air-gapped system, because there's plenty of technologies that jump air-gaps, and we know that big governments (like ours) use them to spy.
It does not matter whether complicated identification schemes involving fingerprints and complex PINs are used to verify identities. Every end of the system is vulnerable to cyber-attack; the browsers, the software, the processing, and even the commands you type into the computer to register to vote. Man-on-the-side attacks, spoof ballots, denial-of-service attacks — there is absolutely no way to create a closed system that would filter out bad code.
It does not matter whether previous (small-scale) experiments have been successful. An American internet election would be a ripe target for hackers belonging to nation-states, criminal gangs, and all sorts of people who spend their days looking for the latest vulnerability to exploit.(How many times has Microsoft had to update Internet Explorer to fix a major bug in the past two years? What confidence could you possibly have?)
It's hard to steal elections conducted in person or with ballots printed on something that isn't made up of invisible electronic bits. It would be much easier to steal, alter, or influence elections that are conducted online. Technology may never advance to the point where our online transactions are safe enough. For some activities, like online shopping, we're willing to allow a margin for error. If someone steals our credit card number and uses it to make online purchases, we'll probably discover it quickly. We gossip about friends because we're pretty sure that they're not going to hack into our caches. We conduct politics online because if someone hijacks our identity, we can get the word out quickly.
Voting, however, is more intrinsically sacred than e-commerce, and really, any of these other activities. There would not be any way to know whether a virus or a hacker changed your vote after you voted, even if you were able to print out a receipt for your vote at home and turn that in for later auditing.
Bubble-in optical scan-voting systems are vulnerable to hacking, but the paper ballot remains intact. You can screw with the computers that read the ballots and screw with the software that counts them, but you can't change the laws of physics, unless you somehow steal paper ballots in advance and treat them with magic disappearing ink that would...actually, I can't come up with even a fanciful way for an election using optically scanned ballots to be stolen or fudged on a massive scale. That's why election supervisors who know their stuff tend to want to use them.
Security is the major concern, but access is another. Until almost every eligible voter has equal access to a computer, internet voting would raise the political power of the connected majority over the non-networked minority; the richer over the poorer, the people who would still have to send in a mail-in ballot or travel to a polling location. Unless the advent of internet voting were accompanied somehow by a mass online enfranchisement, the vote would be unforgivably skewed, and skewed against those who are traditionally screwed by obstacles to voting anyway.
For some small groups, internet voting makes sense. Military computer networks tend to be harder to hack than the regular old internet, and without some type of internet-based balloting, a large number of registered voters overseas might be disenfranchised. Even here, though, the internet is best used to facilitate the distribution of ballots, but not necessarily to receive them or send them back to be counted.
With internet voting, elections could be stolen even before they were held.