A third challenge is the white vote. While winning whites is not as essential as it once was, they will still make up close to 80% of this year’s midterm electorate. Democrats have consistently lost the white vote in recent decades, even in the 2006 congressional landslide. The early polls in 2014 don’t show a changing tide. The Pew Research Center’s February poll showed the GOP with a 53% to 38% advantage in congressional voting intentions among white registered voters.

Then there are the millennials. While support for Democratic candidates among African-Americans and Latinos remains high, young people are less enthusiastic. The Pew center’s in-depth surveys of those ages 18-34 indicate that this generation, a voting bloc so important to Mr. Obama’s two victories, is growing more disillusioned with the president. Millennial self-identification as Democrats has edged down to 50% from a high of 58% in 2009. Pew also found Mr. Obama’s job approval among millennials has fallen to 49% in early 2014, down from 70% in the honeymoon months of 2009, his highest rating among any generation.

Opinion of the president is probably the greatest problem for Democrats this year. At 44%, Mr. Obama’s overall approval rating about matches President Bush’s rating in early 2006 when Republicans lost Congress. And it is not too different from Mr. Obama’s own approval in 2010—45%—when the GOP regained the House.