Pollster Charles Franklin tries to tell a story more complicated than just a "slide":
Based on this estimate, Obama has the second greatest rate of decline, losing 1.6 percentage points of approval each month. Clinton did worse, falling at a monthly rate of 2.3 points. If you look back to the first figure, you see that Clinton fell faster and further, but then rebounded a bit at the end of the summer, making his net change a little smaller (-14) but his overall rate of decline a bit steeper. Obama's chart shows two phases, an initial shallow decline with a bit of a rebound and a more recent decline at a higher rate. Linear fits don't distinguish between these details. But it is pretty clear that based on the linear trend, Clinton fell faster than Obama, contra-Brooks.
What is more interesting is how many presidents managed only slight declines or even gains in the first 8 months. Carter and the second Bush both lost just over a half point per month. Kennedy, Nixon and Reagan all gained, from 0.1 to 0.2 points per month. Eisenhower gained an impressive 0.65 points per month, and the first Bush gained an astonishing 1.9 points per month. So no, not everyone falls off. But yes, Obama has fallen, and quite badly in comparison. Just not quite the absolute worst.
...This chart offers one important corrective to a widely repeated myth: Obama was not "wildly popular" at the start of his term. This exaggeration is repeated so often that it is becoming a universal "truth". But the fact is Obama entered office with strong support, but not so strong as that of Eisenhower or Kennedy, and after a couple of weeks his approval was above average but only by a bit. It is this myth that adds to the drama of his current fall, but the myth exaggerates how high in the pantheon Obama first stood.
The second useful perspective of this chart is that much of the decline has come after 3.5 months, or starting in May. The decline since then has been quite steady, dropping Obama from a shade above average at 3.5 months, to a next-to-last place now. This second slide should be far more worrisome to the White House than the initial polls or the net change from first to last poll.