Political scientist Alan Abramowitz makes a point about shifting party alignments:
Fifty-two percent of Americans now identify with or lean toward the Democratic Party while only 39 percent identify with or lean toward the Republican Party. [Gallup]
The fact that Democratic identifiers now decisively outnumber Republican identifiers means that in order to win, Democrats only have to unite and turn out their own base. If Obama wins the national popular vote by even a single percentage point, it’s worth remembering, he’ll almost certainly win the electoral vote as well. In order for John McCain to win, on the other hand, Republicans not only have to unite and turn out their own base, which they have been fairly successful at doing in recent elections, but they also have to win a large majority of the small bloc of true independents and make significant inroads among Democratic identifiers, which they have not been very successful at doing recently.
The logic appears sound, but polling data belies the claim. The latest Cook Report poll has a partisan breakdown – 50% Democrats, 39% Republicans – similar to the Gallup numbers Abramowitz cites. Against Obama, McCain wins independents by five points, hardly a “large majority,” and captures 12% of Democrats, hardly a “significant inroads” since that’s just a hair above the Democrat defection rate (10% or 11%) for the last four presidential elections. Despite not fulfilling Abramowitz’s conditions, McCain loses the general election to Obama by only one point, 44% to 45%. Just a tiny improvement among independents and McCain secures the White House.