Democrats are facing a senior problem that could get even worse this year. The party has traditionally had trouble with older voters, losing the group aged 65 and over by 21 points in 2010 — when Republicans picked up 63 seats — and by 12 points in the 2012 presidential race. Seniors are the GOP’s most reliable voting bloc in midterm years, turning out in higher numbers than Democratic base voters. A recent Gallup poll showed seniors have become even more Republican over the last two decades; in 2013, 48 percent considered themselves Republican. That spells trouble for Democrats, who...