Kerry has to win Ohio to win the race.
Democrats are holding out because there is a very small chance that the provisional ballots in Ohio could make the difference. However, even James Carville (a Democratic analyst) has said that this the equivalent of expecting to win two inside straights in a row.
Bush's lead in Ohio is increasing (about 140,000 right now), and his lead is at least half the uncounted and provisional ballots uncounted in Ohio. To think Kerry could win Ohio (while not mathematically impossible), is extremely unlikely, to put it mildly.
Bush has almost certainly won the popular vote and even managed to get over 50% of it (which hasn't been done since 1988). While I don't blame people for holding out hope, we have to expect Bush to be the US president for the next 4 years.
I was thrown by the expectations that the new voters would turn out more for Kerry. That didn't happen. There weren't as many new people voting as expected and those who were new voters were more or less split in their votes. I was calling this election for Bush up until Thursday and then I changed my mind on Sunday because of this. I think I learned my lesson here. Don't expect too much from an unknown quantity.