One of the stories attracting attention today is a Gallup finding that a significant percentage of Democrats would vote for McCain if their candidate was not the nominee. But how much do these numbers really reflect what might happen in the general election campaign?
The last somewhat divisive primary campaign was in 2000, between Bush and McCain. In March of that year, the Pew Center for the People & the Press released a report titled "Bush Pays Price for Primary Victory." Following Bush's victory in the 2000 primaries and McCain's exit from the race, the Pew survey found that 51% of those who backed McCain during the primary campaign would vote for Gore in the general election. Only 44% of his supporters said that they would be casting their votes for Bush. Furthermore, a significant share of Bradley supporters also said that they would be supporting Bush in the general election, including 39% of his independent backers. (The figure below comes from the Pew report).
What is notable is not that Gallup finds that some Clinton and Obama supporters currently say that they may vote for McCain if their candidate loses, but that the number is so low compared to what it was for McCain and Bradley supporters in 2000. Only 28% of Clinton supporters (and 19% of Obama supporters) say they'd defect if their candidate lost, whereas half of McCain supporters were saying the same thing after he lost his bid for the 2000 Republican nomination.
Eventually, many of those McCain backers likely returned to vote for Bush and most of the Bradley backers likely returned to vote for Gore. The hard feelings that existed shortly after the end of the primary eventually subsided as the party unified for the general election. It is likely the case that Obama and Clinton supporters would eventually return to the fold and support the Democratic nominee in the Fall as well. However, the key difference between 2000 and 2008 will be the timing. When McCain lost the nomination, Bush had between 7-8 months to court McCain's old supporters. The Democratic nominee will have less time to do the courting this year. The critical question is how much time will he or she have?
UPDATE: I've posted some additional context on this question here.
UPDATE 2: I've now also looked at survey data from fall 2000 to see how common defectors were once the general election came around.