About 15%, in my opinion.
75 percent, though, aren't conservatives... but just sports fans, rooting for their 'team' without any notion of why. Only about 10% actually think about the issues and based on their reasoning, decide to join the GOP.
As for when it became necessary? Well, when it became necessary. IE: when the GOP nominated Trump as candidate for the general.
And can those 15% be deprogrammed? No. But in four years, there will be less of them due to aging and natural death. So their influence will diminish. Hopefully enough so that they cannot dictate the GOP nomination ever again.