Below is a comparison of turnout percentages by race among the voting-age population, according to the Census Bureau. Watch how white turnout changes between general elections and midterms -- particularly compared to other racial groups. (The groups represented below are white non-Hispanic and black non-Hispanic voters.)
But note this in particular: White turnout in midterms matches Hispanic turnout in presidential elections. That's significant. Even if there were an equal number of white and Hispanic voters in the country, more whites would cast votes.
But there aren't an equal number of whites and Hispanics in the country. Using Census data on the size of the citizen voting age population, here's an estimate of the actual composition of the vote among the four groups isolated above. Since that leaves out voters, we've included the Roper Center's estimated breakdown of the 2012 vote for comparison.
The point being: White voters comprise the majority of the electorate in any year but a larger majority of it in midterms -- by a sizable amount.
Even gender plays to the Republicans' favor today. Over time, women have passed men in turnout. But the gap between the two is much smaller in midterm elections.
Past behavior doesn't dictate what happens in the future, of course. But 2014 will likely be as good an electorate as the Republican Party could hope for -- and perhaps as good an electorate as it will ever see again.
* In the most recent Post / ABC News poll, male likely voters preferred Republican House candidates by a 56/39 margin, versus 49/44 preference for Democrats among women. White voters preferred Republicans 59/35, versus 74/21 Democratic preference among non-whites.