Gallup found in April that Republicans were five points more likely to vote than Democrats. More recent measures, including by the Pew Research Center in June, show Republican voters displaying more intense interest than Democrats. If 2008 stay-at-home Republicans vote, Mr. Obama's margin would shrink by more than one-third (to 6.1 million). Similarly, the 2.4 million veterans who voted in 2004 but did not in 2008 could turn out in 2012. Mr. McCain's winning margin among vets was 10 points.

Nor can Mr. Obama count on winning the support of 9% of Republicans—or roughly 3.7 million—as he did in 2008 (according to exit polls). If he instead wins the same 6% of Republicans as Sen. John Kerry did in 2004, then 1.25 million Obama Republicans would be subtracted from the president's column and added to Mr. Romney's. That would narrow Mr. Obama's popular-vote margin to 3.6 million.

According to the exit polls, Mr. Obama won independents by eight points in 2008 (52% vs. 44% for Mr. McCain). But the July 1 CNN/Opinion Research poll showed Mr. Romney winning independents by seven points, 49% to 42%. The June 24 Gallup poll found Mr. Romney up by one among independents, 43% to 42%. Independents will shift back and forth, but if they split 49% to 49% (with the rest going to minor candidates), then Mr. Obama's vote total would be shaved by 1.1 million and Mr. Romney's would grow by an equal amount, cutting the president's margin to 1.4 million.