But a big chunk of President Obama’s 9.5 million-vote advantage is probably gone. Let’s break this down. According to exit polls, 44.8 million Republicans showed up to vote in 2004 while only 41.4 million did in 2008. Almost all those 3.4 million Republicans who stayed home have been energized by Mr. Obama’s agenda and are now eager to vote against him. …

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.