You got a good discussion here, so let me just say that a reasonable estimate of the change in turnout that would result from online voting is an increase of 2%. That's about the participation increase that accompanies every change— including Motor Voter, which the Democratic Party hoped before 1993 would be a major participation-changer for working-class Americans. Same-day registration, on the other hand, has had much more substantial effects in the jurisdictions that have implemented it. The parties both show little enthusiasm for it, however, from the moment that it elected Jesse Ventura Governor of Minnesota. It isn't just that the party of the upper class always wants to keep working-class turnout down, or that the party in control sees no need to enfranchise the other side's voters. It's also that too easy voting makes organized political parties nervous, because it decreases the returns to organization overall. That only becomes more true under 21st century conditions, where the competitor to a political party on election day can be a flashmob. Their view of political stability, which is the stability of the party system, compels them to maintain friction in the electoral system, just as it continues to keep operating control of US elections in the hands of partisan elected officials and their appointees, which no sensible advanced democracy would do.

How votes are transmitted to the government that are not cast at a polling place is a comparatively small matter against that background. Hybrids of online and traditional snail mail behavior make the most sense. A state, for example, can easily establish an online method for requesting and printing an absentee ballot, with an encrypted 2-D barcode that securely identifies the voter and incorporates information that can be used to prevent fraud. The voter then fills out and mails the absentee ballot as usual. Oregon is using mail-in voting only statewide at present, as you probably know, in all elections. The Republican Party there has not found much traction there for its constant beefing about supposed fraud, which it is always doing everywhere. If mail-in can work statewide, it will be adopted elsewhere, with online components used to drive costs down further. Automated authentication of cryptographically printed absentee ballots will turn out to be one of those solutions. The Post Office already has the necessary technology, which it uses to authenticate postage purchased online.