According to the Politico, a "prominent Republican" says McCain will get an extra 15% of the vote in a race against Barack Obama,

I was talking the other day to a prominent Republican who asked me what I thought John McCain’s strongest issues would be in the general election.

Lower taxes and the argument he will be better able to protect America from its enemies, I said.

Republicans have a pretty good track record with those two.

The Republican shook his head. “You’re missing the most important one,” he said. “Race. McCain runs against Barack Obama and the race vote is worth maybe 15 percent to McCain.”

The man I was talking to is not a racist; he was just stating what he believes to be a fact: There is a percentage of the American electorate who will simply not vote for a black person no matter what his qualities or qualifications.

How big is that percentage? An AP-Yahoo poll conducted April 2-14 found that “about 8 percent of whites would be uncomfortable voting for a black for president.”

I don’t know if 8 percent sounds high or low to you, but I was amazed that 8 percent of respondents were willing to admit this to a pollster. And I figure that the true figure is much higher.

There actually has been a study done on this at the Congressional level,

In fact, white Republicans nationally are 25 percentage points more likely on average to vote for the Democratic senatorial candidate when the GOP hopeful is black, says economist Ebonya Washington of Yale University in a forthcoming article in the Quarterly Journal of Economics. White independents are similarly inclined to vote for the white Democrat when there's a black Republican running, according to her study of congressional and gubernatorial voting patterns between 1982 and 2000, including five Senate races in which the Republican nominee was black.

...But racially motivated crossover voting is not just a Republican phenomenon. Democrats also desert their party when its candidate is black, Washington found. In House races, white Democrats are 38 percentage points less likely to vote Democratic if their candidate is black.

Consider that last sentence,

"In House races, white Democrats are 38 percentage points less likely to vote Democratic if their candidate is black."

That is a daunting number to begin with and there are two factors that make it even more frightening for the Dems.

#1) Obama has been very closely associated -- and continues to be associated with -- a virulently anti-white reverend for more than a decade. Since that's the case, there's going to be a lot of suspicion -- with good reason -- that Obama really hates white people every bit as much as Wright does and hasn't disowned him because he secretly agrees with him. That's very dangerous for Obama because people do not vote for candidates that are hostile to them.

#2) John McCain is a genuine, honest-to-goodness moderate who will be much more appealing to Democrats than the average Republican.

So, what percentage would Obama lose in a general election? That's hard to say, but 15% may not be out of line at all.