One way to look at the 2014 midterms--any midterm, really--is to consider them to be a referendum on President Obama. That is, voters will vote mostly based on their perception of President Obama rather than on their perceptions of the candidates running in their district or state. In such a world, where local races are completely nationalized, Democrats are doomed. As of December 2013 (which is a long time ago according to campaign time, but recent enough to make some judgments), in only one state--his native Hawaii--is his approval rating above 60%; moreover, of the 12 most disapproving states, two, Alaska and Arkansas, are home to two of the most vulnerable Democratic senators in the country.
Of course, we don't live in that world. Regardless of whatever pundits might say about how the president's favorability impacts the races in the states, the fact is that the elections this year don't have President Obama vs. a generic Republican on the ballot. They're about two candidates that have their own records and their own personal appeal on which to run.