Daily Kos/SEIU State of the Nation poll: Despite switch to likely voter model, Obama still up by 6

Public Policy Polling for Daily Kos & SEIU. 9/7-9. Likely voters. MoE ±3.1% (8/23-26 results):

Q: If the candidates for President this fall were Democrat Barack Obama and Republican Mitt Romney, who would you vote for?

Barack Obama: 50 (44)
Mitt Romney: 44 (44)
Undecided: 6 (6)

So where's Obama's convention bounce? You're looking at it! Allow me to explain. This week, Daily Kos and SEIU made an important change to how we conduct our national polling. Specifically, we switched from a registered voter model to a likely voter model'meaning we're now confining our pool of interviewees to those that our pollster, Public Policy Polling, thinks are "likely" to vote in the November elections. Typically, the switch from RVs to LVs is made relatively late in the cycle, because asking someone if they're likely to vote when election day is, say, a year off is a very dicey proposition. (Do you know what you're going to be doing a year from today?) So we felt that changing to likely voters after Labor Day made sense.

This means that our polling from this point forward isn't directly comparable to the surveys we conducted before the holiday. But it doesn't mean all comparisons are out of bounds, and that's where the "bounce" comes in. Nate Silver helpfully explained what switching to LVs typically means in presidential polling:

In the past six presidential election years, the shift to likely voter models has always helped the Republican candidate, but the difference has also always been small, usually amounting to a net of one or two percentage points in the margin between the two candidates.

This research suggests that had we not made the changeover and were still conducting polls among registered voters, Obama might instead have a 7- or 8-point lead. Put another way, the fact that we're now using a more pro-Republican screen nevertheless didn't help Romney. We can't know these things for sure, of course, but I think it's reasonable to conclude (particularly when viewed in light of other polling from other firms) that Obama did indeed get a small bump heading out of the conventions.

And if you're curious to know why likely voters tend to be somewhat more Republican on the whole, that's a topic which itself could consume many blog posts, but Silver briefly explained that as well:

Why do Republicans have this advantage? Because, for many years, the demographic groups that have tended to vote Republican have also tended to have demographic characteristics'for instance, being older, whiter and wealthier'that correlate with having a higher propensity to vote.

As for the mechanics of how PPP susses out likely voters, Tom Jensen described what is actually a very simple change with regard to the state-level polling PPP conducts. It's also applicable to our national polling:

We're going to switch to likely voters for our polls this week. Here's what that entails. Right now when we call people for a poll the introduction is: "This is a short survey about some important Florida issues. We appreciate your participation. If you're not a registered voter, please hang up now." Now the introduction will be: "This is a short survey about the Presidential election in Florida this fall. If you don't plan to vote in the Presidential election, please hang up now." That's the entire shift.

The only difference is that our intro doesn't mention Florida, natch. Anyhow, as always, you can get access to our all our polling, including our archives, on our Weekly Trends page.