You cannot stop people from agreeing to all campaign for a single candidate, but you don’t need to put it in big letters on the ballot. A ballot can just be a list of names. The major parties can inform most voters who they endorse for president. They may also be able to get most people to associate a Congressional or Senate candidate with their party. But, when a voter is looking at their ballot, there is no way that they will know if a party enforced “Smith” or “Jones” for state treasurer, unless the candidate changes their legal name to “Democrat Jones”.

It would probably mean that most voters will just not vote for down ballot candidates, except a very few who brought along the party’s list of recommended candidates or crazies like me who try to research candidates on the ballot.

Also, there is no reason for the state to host and administer primaries for major parties, police that a given voter does not vote in multiple parties’ primaries, or forbid a locally popular candidate from appearing on the ballot just because they are a member of a national party who endorsed another candidate.

I am not sure if it would be the best choice, though, since it would make it more likely that a party insider would get elected (since they are going to appeal to party die hards).