Iowa allows caucus goers to register at the caucus. The newbies just might show up.
Sure. But the pattern is a rush of registrations at the caucus is preceded by a rush of registering traditionally. Anyone who is taking the matter seriously is going to register ahead of time if possible.
There have been a few thousand ‘new’ GOP registrations, but a lot of them are party regulars who switched their registration to Independent a year or two ago as part of protest against the GOP-backed raise in gasoline taxes.