Ron Brownstein has written an excellent piece on the "tectonic plates" driving the 2014 election, but this sentence is particularly clarifying:
From 2008 through 2010, turnout dropped about one-third for African-Americans, almost two-fifths for Hispanics, and fully 55 percent for 18-to-24-year-olds, compared with about one-fourth for whites and only one-eighth for seniors.
Brownstein calls the modern Democratic electorate a "a boom-and-bust coalition" that, in recent years, has dominated presidential elections but melts away come the midterms.
The boom-and-bust coalition is leading to a boom-and-bust Senate. "If Republicans take the Senate tonight, neither side will have held it for more than eight consecutive years since 1980, a period of volatility unlike anything since the late 19th century," Brownstein writes.
And the boom-and-bust cycle is likely to happen even quicker this time. As of now, it looks like Democrats will be defending 10 Senate seats to the GOP's 24 in the 2016 election, and doing it amid the more favorable demographics of a presidential election. So it's likely that even if Republicans win the Senate tonight, Democrats will win it back in 2016.
In the long term, this is one of the Democratic Party's most severe liabilities: they need to somehow convince their voters that congressional elections matter. Otherwise, they're going to win a lot of presidential elections, but they're rarely going to be able to govern.