Another sign that Mr. Obama hasn’t fundamentally changed America’s political structure: Compared with 2008, there were 371,800 fewer white votes cast in Ohio in 2012, when Mr. Obama carried the state by 166,214 votes. Many whites who voted for him in 2008 couldn’t bring themselves to do so again or to vote for Mr. Romney. Their staying home represented a tactical victory for Mr. Obama, not a strategic realignment.

Nor can Democrats count on young people forever remaining Democrats. Voters age 18-29 were those most likely to move away from Mr. Obama between 2008 and 2012, and Republican identification generally increases as people graduate college, start work and begin families. Of those who were 18-29 years old in the 1972 election, 47% were Democrats, 26% Republicans and 28% independents. By 2012, these same voters (now ages 58-69) were roughly 37% Democratic, 34% Republican and 29% independent. They backed Mr. Romney by 51%-47%. …

Even among Hispanics, Mr. Obama hasn’t locked things up. While Mr. Romney received only 27% of Hispanic votes nationwide, he received 32% in the seven battleground states with exit polling (including 42% in Ohio) and attracted more Hispanic votes than John McCain did in 2008 in California (by 8%) and Nevada (5%).