"Al Gore was ahead of George W. Bush by three points right after Labor Day in 2000 and the race ended up being a statistical tie (although Gore received about 500,000 more votes than Bush). Jimmy Carter was 4 points ahead of Ronald Reagan after Labor Day in 1980, but Reagan went on to win by 10 points -- a 14-point swing in the margin between candidates. Carter was also ahead of Gerald Ford by 11 points at Labor Day in 1976, but ended up winning by only 2 points -- a 9-point margin change. Richard Nixon was ahead of Hubert Humphrey by 12 points at Labor Day in 1968, and the popular vote in that race ended up being only a tenth of a percentage point in Nixon's favor -- a 12-point margin swing.
In short, the eight-point margin swing it would take for Kerry to edge out Bush in the popular vote on Election Day is historically well within the realm of possibility"
If I stuck with post-Watergate races (including youth suffrage and campaign-finance reforms), then I should drop 1968 as perhaps demograpically out-of-date. That leaves 1976 R+9 after Labor Day, 1980 R+14 after Labor Day, and 2000 R+3 after Labor Day. As a sample, this is not a set here that looks favorably on Democrats.