Except it's a horrible plan to run as a left wing or right candidate in a state. Look at Todd Akin - he allowed a fairly liberal (for Missouri) Democratic Senator be elected because he went too far for Missouri. Bernie Sanders is far left for the US, but firmly center left by Vermont standards. Ted Cruz is a crazy conservative, but he's center right by Texas standards.
You want to be as liberal (or conservative, if you're on the right) as possible, while still winning the general election. Unfortunately, due to the changing demographics of the Democratic party, the point is way more to the right in a midterm than in a Presidential election.
That's not going to change - the turnout for the midterms have always sucked historically. It's just that 2010 was the first 'normal' midterm since 1994 and as a result, the DNC didn't realize how much the demographics had changed in the past few years because external circumstances had dampened the effect in previous midterms (Monica, 9/11, Iraq War).
Even Barbara Boxer, someone whose liberal credentials are really undeniable, "only" won by 10% in a state that leans left about that much in normal elections.
The Democratic coalition is not a midterm-voting electorate.