Michael Barone: Although Obama's numbers dipped, Romney didn't inspire voters to vote

Both of the elections involved incumbent presidents with approval ratings hovering around or just under 50 percent facing challengers who were rich men from Massachusetts (though one made his money and the other married it).

In both cases, the challenger and his campaign seemed confident he was going to win — and had reasonable grounds to believe so.

In both elections, the incumbent started running negative ads defining the challenger in the spring. And in both elections, the incumbent had at least one spotty debate performance.

In both elections, each candidate concentrated on a more or less fixed list of target states, and in both elections the challenger depended heavily on outside groups' spending that failed to achieve optimal results.

The popular vote margins were similar — 51 to 48 percent for George W. Bush in 2004, 51 to 47 percent for Barack Obama in 2012.

The one enormous difference was turnout. Turnout between the 2000 and 2004 elections rose from 105 million to 122 million — plus 16 percent. Turnout between the 2008 and 2012 elections fell from 131 million to 128 million — minus 2 percent.

Turnout is a measure of organization but also of spontaneous enthusiasm.

In 2004, John Kerry got 16 percent more popular votes than Al Gore had four years before. But he lost because George W. Bush got 23 percent more popular votes than he had four years before.

Kerry voters were motivated more by negative feelings for Bush than by positive feelings for their candidate. Bush voters were more positively motivated. Political reporters had a hard time picking this up. His job rating was weak, but Bush voters tended to have a lot of warmth for him.