Keeping an eye on the generic ballot

A new national Fox News poll shows Democrats leading Republicans on the generic congressional ballot by eight points, 45% to 37%, which is a pretty sizable advantage, and which offers us a good excuse to talk about what makes the generic ballot interesting.
The Huffington Post has a great Pollster feature that aggregates polling on this question, and shows Democrats with a seven-point lead. As the image hopefully makes clear, the Democratic advantage has grown steadily over the last two months, as congressional Republicans shut down the government and sparked another debt-ceiling crisis. (The blue line shows support for Democrats; the red line shows support for Republicans.)
But generic-ballot polls are not without flaw. Likely voters are asked for their general partisan preferences when voting for members of Congress without specific names of candidates. It's why it's called a "generic" ballot -- respondents are saying whether they're inclined to support a Democratic or Republican candidate without actually knowing anything about those candidates themselves.
Perhaps more important is the size of the advantage. If Democrats lead Republicans on the generic ballot by a couple of points, that's nice for Dems, but it's almost certainly insufficient -- it's not a level playing field, and the Democratic advantage would have to be much greater to overcome impediments like gerrymandering.
How much greater? Well, that's tricky, and even professional number-crunchers disagree. Political scientist Nicholas Goedert made the case this week that if Democrats go into the 2014 midterms with a lead on the generic ballot of 5 or more percentage points, they stand a pretty good chance of winning back the House. Nate Cohn, however, recently argued the Dems' advantage would have to be closer to 10 points.
I can't speak with authority as to who's model is better, but this at least offers us some guidance when these polls are released.
House Speaker John Boehner (R-Ohio) told reporters this week, in reference to the midterms, "I think we're going to be fine."
Given the recent evidence, I wouldn't be so sure.