Utterly terrible. Party members are the only people that should have a say in who their party runs for office.
But who is a party member? Any voter can sign up for a party and just as easily sign up for a different party (or none at all) after an election is over.
Then get rid of state-mandated registration and let parties control who they admit and their own requirements for membership. The Democrats and the Republicans alike should be able to manage their own membership, rather than allowing the state to do so for them. This is how European parties and American third parties already work.