Brian Schaffer, who on Wednesday reminded us of some Pew Research Center results from March 2000 on defectors from presidential candidates supported in primaries, has done one better. He went back to the data from the Annenberg National Election Study (NAES) in 2000 and checked the defection rates of supporters of John McCain and Bill Bradlee during the fall campaign:
How common were defections? Fairly common, actually. Even in October, only 49% of former McCain voters intended to vote for Bush and 29% were planning on casting their ballot for Gore (in March of 2000, a Pew Survey reported that 51% of McCain supporters planned to vote for Gore). McCain supporters were also far more likely to be undecided late in the race as 11% of this group reported that they still did not know who they intended to vote for.
Former Bradley supporters were also divided. While 52% of this group planned on voting for Gore, another 28% intended to vote for Bush.
Perhaps unsurprisingly, both former Bradley and former McCain supporters appeared more likely to prefer 3rd party candidate Ralph Nader. About 10% of Bradley supporters and 7% of McCain supporters expressed their intent to vote for Nader.
Schaffner also notes that since "since turnout in primaries is usually far less than it is in general elections," the defectors made for relatively small portion of those that ultimately supported Bush or Gore:
Based on this survey, former McCain voters accounted for 4% of those who intended to vote for Gore while former Bradley supporters accounted for 2.6% of those who voted for Bush. (Interestingly, citizens who voted for Gore in the primaries accounted for 1.8% of those who intended to vote for Bush and Bush primary supporters were 1% of those who intended to vote for Gore).
Of course, as both Schaffner and many of our readers point out, 2000 is not 2008. Both party primaries were wrapped up fairly quickly. How big a factor defections will be in 2008 may depend on how long it takes to resolve the Democratic nomination fight.