One way of gaming out what is happening in this election is to compare George W. Bush’s approval rating in 2004 with Barack Obama’s approval rating now.

So where are we? Well, Obama continues to track Bush, but is running just slightly behind him. So consider that a warning for overconfident liberals — or conservatives in premature despair. By this metric, Obama is on track to win, but just barely.

The numbers, per Gallup. Obama is sitting at 50 percent approval in their current (September 25-27) reading, and has been at either 50 or 51 for each of the last six days. That’s four (or three) points behind where Bush was at the time; he had reached 54 percent approval over September 24-26 in 2004. But Gallup didn’t run a constant tracking poll on approval back then, and both of Bush’s other September numbers were at 52 percent, and he fell to 50 from October 1-3 — just where Obama is.

Bush, like Obama this year, had received a convention bounce that stuck for all of September, but he then fell spent most of October back just under 50, where he had been in August. It wasn’t much, but it was enough for him to win by just under three percentage points.

The reason it’s a good idea to compare Obama today to Bush in 2004 is not that it guarantees a prediction of the outcome. Rather, it’s because there’s a pretty good relationship between approval ratings and re-election results. And Bush is the only elected president in the polling era to have a situation like that of Obama, in which he is approaching his re-election date without an approval rating that clearly signals a win (Ike, Nixon, Reagan, Clinton) or a loss (Carter, George H.W. Bush).

It’s worth noting that the general event environment in 2004-2005 (the deepening debacle in Iraq) was gradually pushing Bush lower, while the general event environment this year so far (rising economic optimism) has been gradually moving Obama higher. But of course things could change at any moment.

As we move into October, the head-to-head polling begins to be a better predictor of the final results, but approval ratings do a nice job, too. It’s unlikely that any president with better than 50 percent approval will lose, and unlikely that any president below 45 will win, no matter what the out-party is doing. Bottom line? By this measure, Obama continues to be the favorite. But if he’s really just a bit behind George W. Bush in 2004 and stays there, that could mean he’s heading towards an election night that could be longer than the current horse race polls have it. In short: Obama is on track to win, but don’t get overconfident.