GOP fortunes lie with the independent voters. In a mid term election independents will come out if they think the country is in a world of hurt and they are personally suffering (i.e financially). If gasoline prices go low and the economy is strong, why should the independents come out? If that is the case, then it all comes down to which party can turn out their voters. If I hear that we have heavy turn out in my district, then the independents are motivated to come out in a midterm election (meaning bad news for the incumbent).
I think base turnout is big also.
Actually, the problem for RATS is that a large percentage of their base doesn't show up for mid-term elections. They win mid-term elections when Republicans don't show up, and they have a measure of success in on-term elections due to voter fraud.
Every election, of course, renders new data and new analysis, but I will remain convinced until disproven that we win as long as we show up and overcome fraud sufficiently. Maybe not every seat, but sufficient numbers.