Palin saved GOP from landslide: Morris
Dick Morris ran President Clinton's successful 1992 presidential effort and is considered a pretty fair political analyst (although he was convinced that Hillary Clinton would be the Democratic nominee, guaranteeing her victory over Obama).
Nevertheless, Morris makes a strong case that without Sarah Palin on the ticket, the Republican rout would have been much worse:
Sarah Palin made a vast difference in McCain’s favor. Compared to 2004, McCain lost 11 points among white men, according to the Fox News exit poll, but only four points among white women. Obama’s underperformance among white women, evident throughout the fall, may be chalked up, in large part, to the influence of Sarah Palin. She provided a rallying point for women who saw their political agenda in terms larger than abortion. She addressed the question of what it is like to be a working mother in today’s economy and society and resonated with tens of millions of white women who have not responded to the more traditional, and liberal, advocates for their gender.
Another argument could be made that despite the fact the overall Republican turnout declined from about 30% of the electorate to 28%, it would have been much lower if Palin were not on the ticket. She didn't convince every Republican to vote for McCain. But there is little doubt more Republicans voted for him than would have otherwise.