It's not that simple. I represent myself better than anybody else; I don't run, because I know I'd never win. Similarly, I would argue that by and large that holds true for most people: they are best capable of speaking for, and representing their own views, but don't run for office either out of apathy or because they have assessed their chances of winning as somewhere between slim and none.
Supporting a candidate therefore is not just a matter of 'who best represents me,' but electability has to figure somewhat into the calculus. Hypothetically, suppose there was a candidate who represented you with 100% agreement but didn't have a snowball's chance in hell of winning even with all your support, and a second candidate who only represented 85% of your views but would have the election locked up if they obtained your support.
Note that I'm not drawing any parallels to any candidates in the GOP right now regardless of what you or others may infer from the above. I'm simply saying that unless we as individuals throw our own hat into the ring, we are ceding our representation to somebody else, and in doing so must (if was have any brains) throw our support to somebody we believe is electable. It has to be part of the consideration.
Respectfully, that’s exactly why we no longer have representative self-government, why our politics has devolved into political bookie-ism, and why generally the GOP ends up nominating someone that the base of the party viscerally hates.