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House Forecast: G.O.P. Plus 54-55 Seats; Significantly Larger or Smaller Gains Possible

Republicans are well-positioned to win control of the House of Representatives in tomorrow’s elections, and quite possibly to achieve the largest gain made by either party in a Congressional election since World War II.

Our forecasting model, which is based on a consensus of indicators including generic ballot polling, polling of local districts, expert forecasts, and fund-raising data, now predicts an average Republican gain of 54 seats (up one from 53 seats in last night’s forecast), and a median Republican gain of 55 seats. These figures would exceed the 52 seats that Republicans won from Democrats in the 1994 midterms.

Moreover, given the exceptionally large number of seats in play, the Republicans’ gains could be significantly higher; they have better than a one-in-three chance of winning at least 60 seats, a one-in-six chance of winning at least 70 seats, and have some realistic chance of a gain exceeding 80 seats, according to the model.

However, the same factors that could provide Republicans with extraordinarily large gains if their turnout is strong tomorrow could also cut against them if Democrats turn out in greater numbers than expected, or if the polling has underestimated the Democrats’ standing.

Of the projected Republican gains, many are tenuous. If we allocate all 435 seats to the leader projected by our model — no matter how slim the margin — Republicans would net a gain of 59 seats. In 15 of these 59 seats, however, the Republican is projected to win by fewer than 2 points. It is likely that Republicans will lose at least some of these — which is why the model forecasts an average gain of 54-55 seats, rather than 59, when looking at the seats on a probabilistic basis. If they lost all of them, however, their gains would be merely 44 seats, which would put the Democrats within striking distance of retaining the House. And if Democrats beat their projected margins by about 3 points across the board, they would be about even-money to keep control of the House.

Such broad-based errors have occurred before in midterm elections. In 1998, essentially all key indicators predicted significant Republican gains, but they actually lost 5 seats to the Democrats. In 1994, on the other hand, many indicators pointed toward a total Republican gain of 20-30 seats, when they in fact netted 52 from Democrats.

Our model thinks there is a 16 percent chance of a polling and forecasting error large enough to allow Democrats to keep control of the House; this figure is unchanged from earlier this week.

Still, absent such a mistake, Republicans will have much to celebrate. They are favorites to pick up seats in essentially all regions of the country, with the possible exception of the Pacific Coast. Longtime Democratic incumbents like John M. Spratt Jr. of South Carolina and Earl Pomeroy of North Dakota are likely to lose their seats, and others like Raúl M. Grijalva and James L. Oberstar now have a 25-30 percent chance of doing so, according to the model. Some other Democratic incumbents, like Barney Frank of Massachusetts and Dennis J. Kucinich of Ohio, appear to be considerably safer, but could fall in a wave that exceeded 70 seats.

We do not perceive any particularly dramatic nationwide movement toward one or another party in the closing days of the election. There is certainly no evidence in the polling that Democrats have closed their deficit with Republicans, but claims that Republicans have expanded their margin are also tenuous, as generic ballot trends have been roughly flat since Labor Day.

However, it has become abundantly clear that few if any Democrats will be spared a serious challenge from Republicans, and recent polling has suggested that some who had seemed to be relatively safe — particularly in states like New Mexico, Iowa, Maine and Rhode Island, where gubernatorial races have broken against Democrats — may in fact be quite vulnerable. It is possible and perhaps somewhat likely that a few Democrats in districts that have received little polling and little attention will fall. As a slight counterpoint to this, districts containing Republican incumbents, some of which might ordinarily be vulnerable, have also received little attention from pollsters and Democrats could win a few of these, even if they are having a fairly poor night over all.

This will more likely than not be the final FiveThirtyEight House forecast of the 2010 cycle, but we anticipate re-running the numbers if significant new polling data becomes available overnight.

Nate Silver is the founder and editor in chief of FiveThirtyEight.