John McWhorter voted for Kerry (which is good evidence that calling him a black conservative is at best inaccurate), but he's urging black voters not to stay monolithic in their exclusive loyalty to Democrats. One thing I really appreciate about McWhorter is his willingness to say what he likes about Bush while disagreeing with him on important issues. He does the same about Republicans in general, and he thinks Republicans favor policies at some times and with some issues that should lead black voters to vote for them now and then. He's insisting that the black voice will not be heard by Democrats if they can rely on the black vote every time without doing anything to earn it, and even liberal black leaders like Al Sharpton repeatedly make the point that Democrats don't have black concerns at heart most of the time.
As a result, black voters are merely the mascot of the Democratic party, as evangelicals have been with Republicans, though if you believe the pundits that might change. One reason why it might be changing is that evangelicals who are hardline conservatives threatened to bolt if Bush went too soft of gay marriage, and many of them did anyway. McWhorter is saying that black voters need to consider Republicans and then vote for them when they have good things to offer, regardless of the racist past of the Republican party (not that the Southern Democrats are any better). I think he's right. The only way black voters' concerns will be listened to and acted on is if their vote is at stake. If black voters were swing voters, as all the other minority groups are, then parties would have to give the dominant mindset of black voters a place at the table.
The same might be true of evangelicals. The issues many evangelicals care about that more moderate Republicans have avoided have now come front and center for the Republican leadership, but there's a catch. If the Republican leadership continues to ignore the biblical concerns for things that conservative Republicans tend not to care about, then many evangelicals will still feel ignored within the Republican hierarchy. Some evangelicals care more about gay marriage and abortion than caring for the world God has given to any government to steward, both in terms of the environment or in terms of the people. Many don't see a hierarchy and want someone who will value all their principles. Since that's harder to come by, picking and choosing will always go by what seems more important at the moment, and gay marriage judicial and mayoral activism and the likelihood of Supreme Court vacancies on a court favoring abortion 5-4 have decided the vote this time around. That may not be so next time.