I don't think that would necessarily be legal- but I think that parties should be able to sue for damages on the material support given to candidates when those candidates switch parties.
Stonewall them when they get in office. Spend more political and financial capital to make sure they don't get elected and don't pass policy. Ensure constituents know they're a cheater. Support primary challenges against them. Go out of their way to make sure that they don't succeed in their new party.
People defecting on single votes isn't really what I'm looking at regulating here- though voting with the party certainly is an element of party loyalty. What I'm saying is that there should be a will among the activist, voter, and donor class of the Democratic Party to make an example of people who do what Cotham did. The fact that she did it proves that we're not doing enough to nail these fucking worms to the wall.
The fact that people like Sinema and Cotham are brazenly cheating Democrat voters proves that the party isn't doing enough to enforce loyalty. These people are worms who should be put out of a job irrespective of the cost and have their careers obliterated.
Generous of you to assume it's not a reaction to ShinigamiEyes