Has John McCain fallen too far behind, too late in the presidential campaign, to overtake Barack Obama?

That is the question facing strategists in both parties three weeks before Election Day. History suggests that the answer is probably so.

Mr. Obama has already made history as the first African-American to become a major-party nominee for president. But his breakthrough represents a wild card that could yield election returns at odds with poll results. Beyond that, Mr. McCain’s hopes rest on capturing the support of undecided voters, as well as shaking loose some voters who support his Democratic rival.

No one, including Mr. Obama’s advisers, says such a turnaround in Mr. McCain’s favor is impossible. But the magnitude of Mr. McCain’s task may leave him depending on a misstep by Mr. Obama or a national security crisis rather than on what he can achieve through speeches, advertising or a winning performance in the final debate on Wednesday.