I used to believe this. Now I am not so sure. Nevertheless the Obama campaign better stash their guy's halo and get digging, if a new U.S.A. Today Poll is to be believed:
Republican presidential candidate John McCain moved from being behind by 6 points among "likely" voters a month ago to a 4-point lead over Democrat Barack Obama among that group in the latest USA TODAY/Gallup Poll. McCain still trails slightly among the broader universe of "registered" voters. By both measures, the race is tight.
There's a thing down in the States called The Bradley/Wilder Effect, where black candidates significantly underperform their polling numbers when it actually comes time for voters, esp. white voters, to cast a ballot. Will it take a few points off Obama's totals on election night, so that even if he is up a couple of points in the nationals he might eventually lose to McCain? Hard to say, as the effect seems to have waved over the years.
However, the GOP is doomed in the House and will lose ground in the Senate. Therefore, it will be very easy for a disgruntled Republican/Indy voters to send a message by casting a split ballot--kicking the bums out in the two houses, but keeping a Republican in The White House. Especially if they don't want to vote for a black but won't admit as much to the pollsters.
And Americans have voted this pattern in the past. The1972, 1984, and 1988 elections saw Republicans send one of their own to the White House while the party lost ground in Congress.