Given the structural demand for a Democratic victory, why hasn’t Barack been able to shake McCain? Here’s Howard Fineman’s theory.

Why hasn’t Obama run away with this?

Because the country remains culturally divided. Because the more it looks like Democrats will score huge gains in Congress, the more worried “soft Republican” voters get. Because McCain has succeeded, in the minds of some of those voters, in raising the hoary specter of “tax-and-spend” liberals. Because Obama hails from a place (South Side Chicago) and background (the son of professional academics) more reminiscent of Democratic losers like Michael Dukakis, Al Gore and John Kerry than winners like LBJ, Jimmy Carter or Bill Clinton. Because some voters remember the hate-filled sound bites of the Rev. Jeremiah Wright.

And, to a degree we cannot measure and may never fully know, because Obama is an African-American—and one with a Swahili name at that.

There is nothing that the staffers here in Chicago can do about any of that at this point. Up on the 11th floor of the office building here, staffers are hard at work. They aren’t thinking about those things. Their campaign manager, David Plouffe, won’t let them. “We expected this to tighten,” one of them said to me a few hours ago.

And so, it seems, it has.