All battles are won or lost before they’re ever fought. The primary election is where a general election battle is either won or lost. The Republican party has developed a bad habit of not learning this lesson. More to the point, I have already heard numerous conservatives asserting that they will not vote for Donald if he is picked over Sarah Palin. That means they would be the ones spoiling the election, not Donald Trump.
I've heard some conservatives in the past say the same thing about Alan Keyes. That doesn't make Sarah a spoiler. It makes some of her supporters babies. As for basing your primary vote on who is going to be the strongest candidate in the general election, nobody really knows, early polling aside. Case in point, all the polls showed Reagan would be the easiest Republican for Carter to beat. They litterally popped champagne in the White House when he won the nomination. So far Sarah has been right about more things than the supposedly more knowegable candidates: eg. Obama just came out for a board to determine what medical treatments will be available for seniors in order to save money; they sure laughed about "Obama death panel" didn't they? And they laughed at Sarah for predicting inflation; pay no attention to $5 dollar gas and $40 an oz. silver.