The usually savvy Matthew Yglesias gets things a bit wrong in this post on the utility of state-level polling:
It's really too bad that the folks behind Five Thirty Eight.com have gone and created such a compelling website based around state-by-state general election polling. It's all really well done and, as such, I can't really bring myself to look away. But this stuff is all really and truly meaningless. Six months ago, no polling showed Barack Obama winning the Democratic race, and no polling showed John McCain winning the Republican race and the general election is about six months away.
The comparison in the last sentence isn't valid, however. Presidential primaries are inherently unpredictable for reasons including the lack of clear ideological differences and the greater importance of perceived viability. General elections, by contrast, can be forecast with a high degree of accuracy.
That doesn't mean that state-by-state polling is the right way to predict outcomes -- previous research has shown that macro-level variables like the state of the economy, job approval of the president, war deaths, and/or the length of the incumbent party's time in office explain most of the variance in the national two-party vote. Yglesias and others should focus on those predictors instead. But UW-Milwaukee's Tom Holbrook did find that spring 2004 polls were reasonably predictive of the eventual outcome. For instance, here's his plot of May polls against state popular vote totals in November:
And here are Holbrook's conclusions about the predictive validity of the data:
[W]hen the polling margin was fairly narrow the outcome was truly up in the air. In fact, across all four months [March-June] the poll result called the wrong winner in 17 of the 36 cases in which Kerry's share of the two-party vote in trial-heat polls was between 47% and 53% (this excludes two cases in which the poll result was tied). These results suggest that we should take the term "toss-up" very seriously. At the same time, the poll result was wrong in only 3 of the 44 cases in which Kerry's poll margin was outside this range.
As for me, I think Douglas Hibbs's forecast that the Democrats will get 53-54% of the two-party vote is a reasonable baseline, though I fear that an anti-Obama backlash will reduce that total by 2-3 points. (The Intrade futures market puts the odds of a Democratic win in November at 62%.)