So they really have people who are supposed to look at the signatures on mail-in ballots and compare them to a previous signature(s) by a voter and decide if the sig is valid? As the former FBI agent says, it's a terrible idea that will just result in valid votes being invalidated. You'd need a large number of people with lots of training and experience--and even then it's incredibly problematic given that people don't sign their names the exact way every time. Signature verification is an effort to solve a problem that doesn't exist and would only CREATE a problem. In an effort to weed out invalid ballots, it would weed out /valid/ votes.
Well, its pretty common to review signatures at a BoE. Most candidates (including incumbents) have to gather a certain number of signatures on petitions in order to stand for election each cycle. I have successfully thrown republicans and dems off the ballot (including one elected, HA!) through petition fights.
There is a lot of fraud in petitioning and this is where Republicans tend to cheat. Dems tend to cheat on the ballots. Usually because its harder outside of urban areas to gather enough petition sigs to get on the ballot and requires door to door canvas. In the city, you can simply set up at a subway stop. Sure, many of the sigs will be garbage (not registered, out of district, wrong party, etc) but since you will gather 5x the minimum, it wont matter. But guys will sometimes fake the sigs because they are too lazy (or underfunded) to go door to door. There was a time when I had a decent side business just running vans of petitioning teams of volunteers who did go door to door. Then we would go through each sheet and make sure the witness and the signature were legit and good.
When you have seen a lot of legit sigs and fraud, you can spot the easy ones pretty quickly. But I have fought ones I KNEW were fraud (I knew someone on the team who did it) but was unable to prove it because the sigs were reasonably close, since they used the registration books.
What the dems to with the mail in ballots is basically the same thing as petitioning. The only thing that is really different is that before, it was hard to get the original ballots to sign and then get them back into the Board without arousing suspicion. The process around mail in balloting and drop boxes, etc has basically removed all of those constraints so now it is easy to do.
Reps have been slow on the uptake because again, they are used to cheating on petitioning, not on balloting - many I have known feel that cheating on petitioning is different than doing so on the ballot since one theoretically gives people more choices, while the other actually makes the choice for them. But legally, its pretty much the same.
Only a few in the party leadership will typically know how this really works and most are ignorant of how the sausage is made.
As time goes on and the corrupt ballot process remains, GOP hacks are likely to lose what little scruples they had and it will be an all out slugfest of who can submit the most fraudulent ballots. Once you hear Dems calling for ballot security again, you will know the republicans have gotten as good at cheating there as the dems have been for years.