Rasmussen and Survey USA are responsible for the vast majority of polling data that we've seen so far on the general election. Fortunately for us, they also happen to be two of the historically most accurate pollsters.
However, as I've alluded to a couple of times, there have come to be some systematic differences in the numbers each Democrat has received from each agency. The difference is particularly notable for Hillary Clinton.
Let's run a couple of straightforward charts. First, these are Obama's numbers against McCain in the most recent Rasmussen and Survey USA polls, in states that both pollsters have surveyed since Super Tuesday.
Obama vs McCain
On average, Obama leads McCain by 1.2 points in the Survey USA polls, but trails him by 1.5 points in the Rasmussen polls -- a difference of 2.7 points. Although I think that difference would hold up as statistically significant if we ran a t-test, it's not a huge gap, especially considering that the Rasmussen surveys are on balance slightly more recent, and therefore more likely to have been impacted by the Jeremiah Wright fallout and so forth. (If we look only at those states that both pollsters have surveyed at some point in March, the gap between the two agencies falls to only about 1 point).
It's in Hillary Clinton's numbers where we see the bigger differences:
Clinton vs McCain
Clinton's Survey USA numbers have been about 7 points better on average than her Rasmussen numbers. Look at how devastating the Rasmussen electoral map would be to Hillary: she's losing Pennsylvania, Michigan, Florida and Ohio; she's losing New Mexico, Iowa, Virginia and Wisconsin by double digits. She's losing "core" Clinton states like Arkansas and New Jersey. By contrast, Survey USA has her tied or ahead in almost all of those states (Iowa and Colorado are the exceptions).
So, bellows the choir, who's right? I would caution against any categorical answer to this question (e.g. "Rasmussen is a Republican hack pollster" or "Survey USA's results are really goofy"). Both of these pollsters know what they're doing. Survey USA's polls were devastatingly accurate in this year's primaries, which is why sit atop our pollster rankings. But primaries and general elections are completely different beasts, and Rasmussen performed better than Survey USA in the 2004 general. There are, however, two important differences between the two pollsters of which I'm aware.
Firstly, as has been helpfully pointed out to me, Rasmussen is already applying a likely voter model, whereas Survey USA includes all registered voters. I'm an agnostic on the issue of likely voter models. On the one hand, you sometimes hear it said that "if everybody turned out, the Democrats would win every election in a landslide". I have no idea whether that's actually true or not, especially since Democrats have started to outperform Republicans among traditional high-turnout groups (e.g. high-education, high-income voters). But there's likely an element of truth in there. On the other hand, there is surprisingly little evidence that likely voter models actually work; indeed, Gallup, the pollster with the most (in)famous likely voter model, ranks toward the very bottom of our pollster rankings. On balance, I would tend not to want to apply a likely voter model this early in the election cycle, so that might be a point for Survey USA.
The other difference is that Survey USA tends to push leaners a lot harder. They quite literally make it hard to say you're undecided, since the way that their call script works, there's a pause of several seconds before the 'Press '9' for Undecided' option comes up. So they're going to pick up more soft support than other pollsters. Pushing leaners may be desirable late in an election cycle -- and may account for Survey USA's superior results in the primaries, where support is generally softer in than in general elections. But just as I don't like applying a likely voter model months ahead of an election, I also don't like pushing leaners this early -- especially if you aren't applying a likely voter model, which means that you may getting people who are both indifferent toward the candidates and indifferent toward the act of voting in general, and forcing them into one or another candidate's column. So, point there for Rasmussen.
What's interesting is not so much in the result, but in what that result tells us about the nature of Clinton's and Obama's support. On balance, Rasmussen is likely to capture the most enthusiastic voters -- people who have turned out to vote before, and people who already established a strong candidate preference. Survey USA will cut from a wider cross-section.
The surprise might be that it's Clinton, and not Obama, for whom we see the big differences in the numbers. I say it's a surprise because the conventional wisdom seems to be that Obama is more dependent on a high turnout, particularly from among groups like blacks and youths that are thought to be unreliable voters. However, if we see the difference less as one of turnout and more as one of enthusiasm, the result becomes easier to understand. With her generally lower favorability ratings, Clinton may generate more lukewarm support; people who would probably vote for her if it came right down to it, but would do so only grudgingly. These voters would tend to get picked up by Survey USA, but not Rasmussen. It may also be the case that there there are more enthusiastic anti-Clinton voters than anti-Obama voters, in which case the same thing may be true.
What it's safe to say is that Rasmussen and Survey USA are presenting two different scenarios for how the election might go. The Rasmussen numbers are generally more strongly correlated with John Kerry's results in 2004. The devastating result for the Democrats is if you start with Kerry's numbers and subtract about 5 points across the board. If Kerry had polled 5 points lower nationally, he would have lost Wisconsin, New Hampshire, Pennsylvania, Michigan, Minnesota, and Oregon, representing 69 electoral votes in total. That would have made the result Bush 356, Kerry 182, which gets pretty close to Michael Dukakis territory. If Rasmussen is right, this sort of result might be somewhat more likely for Clinton than for Obama, who has a couple of regional dipoles of strength (Iowa/Minnesota; Washington/Oregon; Colorado/Nevada) that might hold up even if things went badly elsewhere. Contrary to the conventional wisdom, then, there may actually be more downside in a Clinton candidacy than an Obama one, as she appears to have a higher proportion of vulnerable, lukewarm support in the event of poor GOTV operations or a badly-run campaign.
On the other hand, both Democrats have significant upside as represented by the Survey USA scenario: Clinton by sweeping the Kerry states and bringing in Southern states like Florida, Arkansas, West Virginia, and perhaps Missouri and Tennessee; Obama by doing the same but adding a number of states in the Interior West and the Prairie regions, as well as the increasingly "Northern" Virginia.
If you look at the latest fivethirtyeight electoral vote distribution chart, you can actually see this distinction: Obama's edge over Clinton comes more in averting downside scenarios than securing upside scenarios. According to our model, Clinton has a 17.6% chance, but Obama just an 8.5% chance, of winning fewer than 150 electoral votes. On the other hand, Clinton has a 15.5% chance of winning 350 or more electoral votes -- essentially the same as Obama's 17.0%.
For the time being, however, both Democrats need to tie down their core support before we can talk about 350-EV scenarios.
(cross-posted at fivethirtyeight.com)