Jonathan Bernstein notes that Palin has been unable to grow her base:
Palin to date has been a factional candidate, representing little more than a personal faction. And while Jimmy Carter won a Democratic nomination as a factional candidate with a personal faction, that’s been a losing strategy for decades now. Presidential nominations are generally won by coalition-building, not factional, candidates. So while she can’t yet be counted out, unless Sarah Palin shows a so far unseen ability to expand her support, do the things that normal presidential candidates do, and run as a coalition building candidate, she’s relatively unlikely to be nominated.