It's been a significant trend that has frustrated Republican efforts to become the majority party: since the late 90s, a significant number of voters who used to vote Republican for fiscal and/or national security reasons have become turned off. Some of them (at least in 2004 and 2006) voted Democratic. But most are abstaining from voting for any major-party candidate in general elections. Discrepancies in vote tallies in 2000 and 2004 between congressional and presidential votes seem to reflect this, as did the Ron Paul candidacy. Whether McCain can overcome this trend in 2008 remains to be seen.

But this year, I'm detecting something similar happening to Democrats. (See here and here, for example.) Obama's failure to win a majority of the Democratic primary vote has set the Democrats up for trouble. Some of these voters will vote for McCain; others will just abstain. The 2008 general election will thus feature two growing blocs of the disaffected, adding a multidimensional wild card to the outcome.