In the House, the Democrats’ big problem is that their coalition is excessively concentrated in urban areas. This provides Republicans an inherent advantage that they magnified with their control of congressional redistricting after 2010.

In the Senate, the Democrats’ parallel problem is that their new coalition is barely a whisper in smaller, rural, older, preponderantly white states that under the Constitution all receive the same number of senators as behemoth states such as California, Texas, and Florida. This year’s battle for Senate control will almost certainly turn on whether a few Democratic incumbents in Republican-leaning states can overcome that challenge…

Except in North Carolina, there simply are not enough “ascendant” voters in those states to produce a majority; Democrats can win only by capturing enough of the older, blue-collar, and rural whites who have abandoned the party at an accelerating pace under Obama. “The election is being played on Republican turf because of the dominance of noncollege whites and cultural conservatives in the states that are most critical for determining Senate control,” says GOP pollster Whit Ayres.