According to this review of the 2008 Election results and the polls, there’s at least some evidence that the answer appears to be yes:
John McCain’s August 29 announcement of Palin as his running mate surprised the Republican establishment, the media, and especially voters. She made a strong first impression: she enjoyed high approval ratings after her acceptance speech, and the percentage of voters saying that they intended to vote Republican skyrocketed. But within days of the speech, her ratings began a precipitous slide from which she—and the McCain campaign—never recovered. Throughout the rest of the campaign, vote intentions were closely tied to Palin’s approval ratings: each major Palin approval drop was followed, within a day or two, by a drop in McCain vote intention. No other factor moved McCain support with such precision. Comparison of the correlation between running mate approval ratings and vote intentions from 2000 and 2004 confirms Palin’s peculiar importance in 2008.
As for why Palin may have done what no Vice-Presidential running mate has done before — decide an election — the authors point to two factors:
First, John McCain’s age and medical history may have played a role, as there was a reasonably high probability of Palin becoming President before 2012. Second, borrowing powerful themes from Hillary Clinton’s primary campaign, McCain sought to emphasize Obama’s lack of executive experience, but by choosing Palin, he may have undermined that argument at a stroke. Is emphasizing Palin’s role in the unraveling of McCain’s post-convention advantage the same as saying she cost him the election?
That’s not saying that McCain would’ve won if he’d selected someone other than Palin, of course. That’s something we’ll never be able to know. What the polls do seem to show, though, is that the Palin selection, while it did provide a much-needed boost to the McCain campaign in the early days of September 2008, ultimately did far more harm than good.
H/T: Peter Suderman