There's a cheesy 7:37 educational video on Oregon's vote-by-mail process at:
http://www.mcelections.org/
(click on the link above the video tape icon).
Simply put:
1. Ballots are mailed to registered voters.
2. Voters fill out the ballots, put them in the bar-coded return envelope, and sign the envelope.
3. Election officials review the envelope:
a. If unsigned, and time allowing, they mail them back and tell the voter they forgot to sign it.
b. If signed, the signature is verified against the signature on the voter registration card. If not kosher, it's not counted.
This review process is done over a period of two or more weeks as the ballots arrive, so it's not a rush job. Except, of course, the last day, as plenty of people prefer to do it "the old-fashioned way", and drop their ballots off at drop sites.
There has been no controversy since it was approved in 1998.
To commit voter fraud, one would have to:
1. create convincing false voter registrations based on false IDs and unique, valid, physical (can't be "Mailboxes are Us") mailing addresses, and then
2. follow through when the actual ballots arrive, linking up those same false signatures with each valid, physical mailing address without drawing any attention to the scheme.
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As to the low voter turnout so far, maybe since Oregon's gone even "Bluer" than ever, the Democrats are sanguine, and the Republicans are demoralized.
That said, I'm perplexed because the Smith-Merkley U.S. Senate contest is close, and nasty...maybe the nastiness has also turned off additional voters who are not sanguine or demoralized?