Maybe Barack Obama felt he couldn’t afford to give the correct answer.

He was asked at a fund-raiser in San Francisco about his campaign’s experiences in the run-up to next week’s Democratic primary in Pennsylvania. One of the main problems, of course, is that he hasn’t generated as much support as he’d like among white working-class voters.

There is no mystery here. Except for people who have been hiding in caves or living in denial, it’s pretty widely understood that a substantial number of those voters  in Pennsylvania, Ohio, West Virginia and elsewhere  will not vote for a black candidate for president.

Pennsylvanians themselves will tell you that racial attitudes in some parts of the state are, to be kind, less than enlightened. Gov. Ed Rendell, Hillary Clinton’s most powerful advocate in the state, put it bluntly last February: “I think there are some whites who are probably not ready to vote for an African-American candidate.”

This toxic issue is at the core of the Clinton camp’s relentless effort to persuade superdelegates that Senator Obama “can’t win” the White House. It’s the only weapon left in the Clintons’ depleted armory.