The way pollsters decide who is a likely voter -- and the way some pollsters exclude new registrants from their likely voter pool -- can have a big effect on the results those pollsters get. Don't just take our word for it: Check out what the Harris Poll has to say on the subject. Harris is out with a new poll this morning, and its explanation of its results is unusually frank:
"With only two weeks to go before the election, a new Harris Poll finds President George W. Bush leading Senator John Kerry, but the size of the lead depends on how we define likely voters. . . .
"Using one definition of likely voters, those who are registered to vote and are 'absolutely certain' to vote, the poll shows President Bush with a modest two-point lead (48% to 46%). Using this definition but excluding all those who were old enough to vote in 2000 but did not do so, President Bush has a commanding eight-point lead (51% to 43%). This second definition has proved more accurate in the past, but there are some indications that in this election many people who did not vote in 2000 will turn out to vote, in which case it would be wrong to exclude them.
"Adding to the confusion about how to define likely voters (and Harris Interactive has not yet decided which definition to use in our final predictions) this poll suggests that Senator Kerry may be doing better in the swing states, in which the battle for electoral college votes will be decided.
"In 17 swing states (where the total popular vote was tied 48% to 48% in the 2000 election) this poll shows Senator Kerry with a seven-point lead using one definition of likely voters (51% to 44%) and a tie using the other definition (47% each). While these numbers should be treated with caution because of the small sample sizes, they suggest the possibility that the popular vote and the electoral college vote may divide differently, as they did in 2000."