So I see that McDaniel is trying to use a Jim Crow law that is somehow still on the books in MS that states if you vote for someone in a primary you must vote for that same candidate in the general election. Yes, this law is technically unenforceable, at least nowadays. The secret ballot makes it impossible to know how someone voted, so unless you are a mind reader, there is no way to enforce such a stupid law.
However, I'm sure that this law was not intended for the secret ballot. While it is a MS law -I don't know all the voting irregularities there in the Jim Crow past, there have been ways for Southern politicians to know how someone voted, especially in smaller communities. In some parts of Georgia, there was no secret ballot. I don't have a link to this information, but it was reported on NPR several years ago about the lengths that Georgia politicians went to commit voter intimidation. And this was not just intended for black voters.
In smaller Georgia communities, the local politicians ended up running the local post offices. It would be at the post offices that elections were held, and the voter had to write out his choice in front of the local politico on a large table for all to see. And if the voter didn't vote "correctly," there would be consequences. Little things like government checks would not find their way to your home for one. If this wasn't bad enough, I'm sure that other threats or retaliation were employed to punish wayward voters.
I'm sure this creative idea of voter manipulation was not just particular to Georgia. If a corrupt Georgia politician can come up with a way to violate voting rights by not having a secret ballot, I'm sure the same method probably occurred to MS politicians as well. Thus, the MS law for ensuring the primary voter must vote the same way in the general was probably born. It is just a hypothesis though.
Unfortunately, I think the Tea Party may make the secret ballot the next target in their insane campaign to "take America back." They have already shown their antidemocratic agenda with their repeal the 17th Amendment movement, so I'm sure after this "stolen" election that Tea Party loons will insist on the elimination of the secret ballot. Granted, I could be wrong, but the Tea Party is growing more deranged each day.