She’s as credible as Reagan was. I seem to remember he got elected just fine.
She may be as credible as Reagan to you, but I'd bet many people don't believe that a person who served as governor of a sparsely populated state for two years is as qualified as one who served as governor of the most populous state for eight years.
I like Sarah a lot, but after the debacle of the current inexperienced president, I suspect plenty of people will want someone with much more experience in elected office.