That's an interesting issue. I can see why you would want that, but I'm not sure how I feel about it. I quit the GOP earlier this year in protest and am now a registered "decline to state" voter -- i.e., independent without belonging to any party. If the GOP doesn't allow me to vote in the primaries with them, then I can't vote. In California, the Dems will allow independent voters to vote with them, as will most other parties. I wonder if that will matter in the primaries? What are your thoughts?
Note that not all non-GOP voters who vote with Republicans in the primaries are "liberal".
If the GOP doesn’t restrict primary voting the left can game the system. They did it with McCain, and will do it again if the opportunity presents itself.
No, just the vast majority of them. Large numbers of Democrats register independent, especially in the Northeast, just for this sort of mischief.