Mr. Bush was hitting the vital 50% mark in almost half the polls (unlike Mr. Obama) and had a lead over Mr. Kerry twice as large as the one Mr. Obama now holds over Mr. Romney. So why was the 2004 race “a dead heat” while many commentators today say Mr. Obama is the clear favorite?

The reality is that 2012 is a horse race and will remain so. An incumbent below 50% is in grave danger. On Election Day he’ll usually receive less than his final poll number. That’s because his detractors are more likely to turn out, and undecideds are more resistant to voting for him…

Both candidates have advantages as the race enters its final month. Mr. Obama is slightly ahead (but short of 50%). Late-deciding independents will probably break more for Mr. Romney. Clear-eyed operatives in Boston and Chicago know this and are only playing head games with their opposition when they assert otherwise.

Team Obama’s relentless efforts to denigrate Mr. Romney as a sure loser appear to have convinced the Republican candidate that he must run as the underdog. This will make the naturally cautious Mr. Romney more aggressive, energized and specific about his agenda in the campaign’s closing weeks than he might have been. It will also make his victory more likely. America likes come-from-behind winners.