from your link
Gans attributed the GOP downturn to three factors: 1) John McCain’s efforts to unite the differing
factions in the Republican Party by the nomination of Governor Sarah Palin as vice-presidential
nominee was a singular failure. By election time many culturally conservative Republicans still did
not see him as one of their own and stayed home, while moderate Republicans saw the nomination
of Palin reckless and worried about McCain’s steadiness. 2) As events moved towards Election Day,
there was a growing perception of a Democratic landslide, discouraging GOP voters. 3)