My NationalJournal.com column for the week is now online. It revisits the nearly two-week old USA Today Gallup poll that showed a big difference between registered voters and those selected as "likely voters" with a focus on the age of the likely voter pool.
After you read the column, the following data may be of interest. First, notice that while the most recent, conducted in late July, showed a net shift of seven points between registered and likely voters, no such gap existed in the poll conducted just a month before. In mid-June, Obama led by six percentage points among both registered and likely voters.
What makes that difference interesting is the additional data generously provided by Jeff Jones of the Gallup organization showing how respondents in different age groups answered the four questions used to identify likely voters. As noted in the column, younger voters tend to score lower on all four questions. Notice that the percentage of 18-29-year-olds who said they had given "quite a lot of thought" to the election plummeted from June (60%) to July (45%). Similarly, the percentage who rated their chances of voting as a 9 or 10 on a 1-10 scale dropped ten points (from 69% to 59%).
Update: Nate Silver has additional thoughts. Note that the method he describes as "the most logical way to handle" the likely voter problem is, in essence, the way the CBS/New York Times poll will model likely voters in October. Their most recent release provides results for registered voters, but not likely voters.
Also, see the related comments we just posted from Time/SRBI pollster Mark Schulman.