Your point on demographics is well taken. In the last presidential election, MR defeated BHO by about 20 points among white voters. Several decades ago, that fact alone would have been enough to guarantee a landslide victory and enough to win all but a handful of states.
Changing demographic factors help explain why the Dems have won the popular vote in five of the last six presidential elections.
Midterm elections, however, are completely different than presidential elections. Demographics are different. The electorate in midterm elections tends to be smaller, older, whiter, more disciplined, more conservative, therefore more Republican than is the case in presidential elections.
Further, the party out of power usually makes gains in midterm elections, and this tends to be especially pronounced in the 6th year of an incumbent administration. In addition the Dems have the added problem of having to defend so many US Senate seats in red states easily carried by both McCain and MR where BHO’s poll ratings are even lower than the national average.
Historically, that has been true. But the past is not necessarily an indicator of future results. The Dems have developed a very sophisticated and powerful GOTV machine. I can tell you that here in VA, Ed Gillespie has no chance of defeating Mark Warner. And anyone who thinks that defeating Dem incumbents will be easy, is delusional.
I fear the Dems aided by the MSM are setting up the GOP for a major fall. They have created the expectation of a giant GOP wave and when that is not achieved, even if the GOP realizes modest gains, it will be portrayed as a defeat.