by Patrick Appel
Nate Silver parses this Gallup poll, which gives Republicans a 10 point advantage, "higher than any previous midterm Republican advantage in Gallup's history of tracking the generic ballot, which dates to 1942":
Gallup’s survey and some other generic ballot polls are still polling registered rather than likely voters, whereas its polls of likely voters are generally more reliable in midterm elections. At FiveThirtyEight, we’ve found that the gap between registered and likely voter polls this year is about 4 points in the Republicans’ favor so a 10-point lead in a registered voter poll is the equivalent of about 14 points on a likely-voter basis. Thus, even if this particular Gallup survey was an outlier, it’s not unlikely that we’ll begin to see some 8-, 9-, 10-point leads for Republicans in this poll somewhat routinely once Gallup switches over to a likely voter model at some point after Labor Day unless Democrats do something to get the momentum back.
Blumenthal's read is similar:
[Political scientist Joe] Bafumi and his colleagues estimated their 50-seat gain for the Republicans assuming a two-point advantage for Republicans on the generic ballot, which they project will widen to a six-point lead by November. If the Republican lead on the generic ballot is already that wide (or close), their projection for the Democrats would worsen.