It's a thing that actually happens:
https://www.nbclosangeles.com/news/local/cudahy-officials-co...
https://www.dothaneagle.com/news/crime_court/woman-convicted...
https://www.stltoday.com/news/local/metro/cahokia-village-tr...
There are also even more small time cases like this:
https://gvwire.com/2019/08/23/mexican-man-who-supports-trump...
Where it's only one person voting when they're not eligible. Those cases often aren't even prosecuted, but at scale it adds up.
Edit: clarity while trying to maintain brevity.
For a start in California mail in ballots have to be signed and the signature has to match the registered voter's signature on file.
So you're assuming someone can steal a bunch of registered voters' ballots and fake their signatures.
Grave ballots would require new/additional votes. That would sure the expected ballot returns.
There are a ton of ways to verify elections statistically that you could read into
But that's the objection. Mail in voting is problematic because the fraud is so hard to detect.
Suppose someone obtains and submits a bunch of mail in ballots. Ballots of people who don't normally vote etc. How would they even get caught? "We haven't caught very many of them" is the problem.
> The only reason to oppose mail in voting, much like supporting rejiggering districts (gerrymandering), is to rig the vote.
You could say it's to prevent someone else from rigging the vote.
Also, this:
https://fivethirtyeight.com/features/there-is-no-evidence-th...
So if it doesn't really affect the balance of legitimate ballots and only makes fraud more difficult, why would somebody be against it unless they're legitimately concerned about fraud?