There's been a shocking amount of weeping and wailing and gnashing of teeth in Democratic circles considering that most polls still show Kerry ahead. Indeed, the just-released ICR poll (see below) has Kerry ahead by 7 points among RVs, a greater margin than the same poll had three weeks ago.

But it's hard to cheer people up when they're determined to be gloomy. Indeed, there's a tendency to seize on "evidence" that supports a gloomy viewpoint without looking at it very closely.

Take the case of Pennsylvania and recent polling data. Democrats seem to be convinced the race there has seriously tightened (see this post by Josh Marshall which is otherwise terrific and level-headed but buys into this particular meme).

What's the evidence for this tightened Pennsylvania race? Disregarding the recent poll by the very Republican Strategic Vision firm, the most recent reputable poll in PA is the Gallup poll of 8/23-26. That poll shows Kerry trailing Bush by a point among likely voters (LVs). I emphasize that this result is among LVs because most PA polls that people have heard about have been conducted among registered voters (RVs). Therefore, comparing these earlier PA polls to Gallup's current LV result is not an apples to apples comparison and tells you nothing about whether and how the race has changed.

Especially when we note the following: the same Gallup poll that has Bush ahead by a point among LVs in the Bush-Kerry matchup has him behind by 5 points among RVs!

In fact, check out the last three reputable poll results from PA among RVs:

8/23-26 (Gallup): Kerry, +5
8/13-21 (Issues PA): Kerry, +2
8/11-16 (Quinnipiac): Kerry, +5

Conclusion: there is no tightening in the PA race once we do an apples to apples comparison.

This is only the latest example of confusion being sown by Gallup's LV model, which has been producing consistently pro-Bush results lately. Indeed, Gallup did a WI poll at the same time as their PA poll and found Bush ahead by 3 points among LVs in WI, but Kerry ahead by the same margin among RVs.

I continue to believe these LV results should be de-emphasized until later in the campaign. It is still too early to put much faith in likely voter screens/models as representing very accurately the voters who will actually show up on election day. There is reasonable evidence that careful likely voter methodologies work well when it is quite close to an election and do fairly accurately capture that pool of voters. But there is no such evidence for LV samples drawn this far out.

Indeed, my understanding is that Gallup, in particular, does LVs throughout the campaign not so much because they believe they are capturing election day voters all along, but more so that they can avoid having to explain sudden shifts in the horse race question as LV data replaces RV data. There have apparently been some problems with this in the past, so reporting both from the very beginning of the campaign eliminates any potential embarrassments along these lines. But that doesn't mean the LV data is any better at this particular point in time--it merely means they're providing it.

In fact, since the sample size for LVs is smaller and since the composition of the LV sample will shift depending on how political developments are affecting interest and intensity levels among different groups of voters, additional volatility and potential bias is built into the LV samples that is not there with the RV samples.

The issue of bias can particularly be a problem if it's true that, say, Democratic-leaning voters tend to tune into campaigns later and are therefore more likely to be screened out of the likely voter sample until close to the election (especially with a tight screening procedure, which Gallup definitely has). If that's true, then an LV sample could perform reasonably well close to election day, as a greater proportion of Democratic-leaning voters finally get screened in, but be quite biased toward the Republicans until then.

That could be what's happening today.

So take a tip from me: always check those RV results. It'll help keep a smile on your lips and song in your heart!