Blacks, Hispanics and gentry liberals tend to live in densely populated urban areas that are hugely Democratic. You see the same effect on a smaller scale in university towns.
Republican voters are scarce in these areas but more evenly spread around in the rest of the country. You can find many 80 percent Democratic congressional districts. You'll have a hard time finding an 80 percent Republican one.
Here one of the reasons the Six Year Rule often becomes relevant. In off-year elections, the president's party tends to be tethered to his record, while the opposition party can field candidates adapted to the local terrain.
That's what House and Senate Democrats did under the inspired leadership of Rahm Emanuel and Chuck Schumer in 2006 and 2008.
George W. Bush carried 255 House districts in 2004. But in 2006 and 2008, he was unpopular, and a gun-totin', tobacco-chewin' Democrat could carry a rural or Southern Republican-leaning district. Many did.
Republicans may have a hard time doing that in 2014, since their primary voters sometime prefer unelectable purist candidates over those adapted to the local terrain. But Democrats will have a very hard time going local because their party is largely defined by the man in the White House.
The outcome could hinge on events that have not happened and decisions that have not yet been made.
Job approval for Ronald Reagan and Bill Clinton in their sixth years was around 70 percent. The Six Year Rule didn't apply.
Obama's job approval is a little over 50 percent now. But this could rise, depending on events. That would improve Democrats' chances for a House majority.
But it could also fall or hover about where it is. In which case House Democrats' road to a majority is uphill.