Nate Silver at FiveThirtyEight.com writes, “While our forecast and a good deal of polling data suggest that the Republicans may win the House of Representatives on Tuesday, perhaps all is not lost for the Democrats. Here’s one possible scenario for how things might not end up as expected.”

Silver suggests that the GOP wave could sweep through the South taking out marquee Democrats like Reps. Alan Grayson of Florida and John Spratt of South Carolina, but diminish a little in the Midwest with key Democratic seats preserved in Ohio, Michigan and Pennsylvania, but continue to roll westward and take down Dems in Colorado but then stall in Nevada and evaporate along the Pacific Coast, where voters have not succumbed to tea bag fever. Democratic victories there could provide a firewall in which Dems hold the Senate and, narrowly, the House.

Admittedly, there is an element of whistling past the graveyard here but Silver’s suggestion that things may not be entirely as they seem is at least plausible.

Even so, if the likelihood that Dems could hold both houses of Congress is out there, why hasn’t it showed up in the polling? Silver lists five possible reasons:

1. The cellphone effect.

This one is pretty simple, really: a lot of American adults (now about one-quarter of them) have ditched landlines and rely exclusively on mobile phones, and a lot of pollsters don’t call mobile phones. Cellphone-only voters tend to be younger, more urban, and less white — all Democratic demographics — and a study by Pew Research suggests that the failure to include them might bias the polls by about 4 points against Democrats, even after demographic weighting is applied.

2. The “robopoll” effect.

Unlike in past years, there are significant differences between the results shown by automated surveys and those which use live human interviewers — the “robopolls” being 3 or 4 points more favorable to Republicans over all, although the effects vary a lot from firm to firm.

3. Some likely voter models, particularly Gallup’s, may “crowd out” Democratic voters.

Gallup’s traditional likely voter model has consistently shown terrible results for Democrats this year, having them down by around 15 points on the generic ballot, which could translate into a loss of 70 to 80 House seats, or maybe even more…

But there is quite a bit of room to critique the poll. The basic potential issue is that Gallup uses fixed turnout targets. For instance, they estimate that 40 percent of the electorate will vote, and then let their respondents fight it out to see who the 40 percent most likely to vote are.

4. Democrats probably have better turnout operations.

Democrats might be able to coax an extra percentage point or two of their vote to the polls, especially in states like Ohio and Pennsylvania where they’ve invested a ton of resources over the years. And in the event where Democratic turnout equaled that of Republicans (it won’t; the point is they might be able to get it a bit closer), they would probably hold the House, even with most independents breaking against them.

5. The consensus view of Democratic doom is not on such sound footing as it seems.

The case that Democrats could do better than expected — not well, by any means, merely better than expected — rests a little more in the realm of what artists call negative space: not what there is, but in what there isn’t. There aren’t 50, or even more than about 25, districts in which Republican candidates are unambiguous favorites. There isn’t agreement among pollsters about how the enthusiasm gap is liable to manifest itself. There isn’t any one poll or one forecasting method that is clairvoyant, or that hasn’t made some pretty significant errors in the past.

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