The 1948 Truman campaign was the last time a President with low approval ratings was able to get back on track and win the election. But, there are critical differences between 1948 and 2004. Part of the reason Truman was unpopular was that his legislative program was stymied by the Republican controlled 80th Congress (both houses went Republican in 1946). In the 1948 Republican platform, they called for a laundry list of popular legislation which they knew would be popular with moderates and burned-out Democrats--of couse, they had no intention of actually enacting this legislation.
So, at the 1948 Democratic National Convention, Truman called their bluff and announced he was calling the "good for nothing 80th Congress" back into session on July 25. He did. The legislation didn't go anywhere, and Truman ran for reelection by running against the Congress. It didn't hurt Truman that the economy was also doing pretty well.
Bush, with a Republican controlled House and Senate, doesn't have that luxury. So, he essentially doesn't have anything to run against, and no excuses for his utter lack of accomplishments.
The economy is mired in a jobless "recovery" that threatens to turn into another recession. Skyrocketing oil prices threaten a return to inflation. The Dow is under 10,000 again. Bush has been in office three years. He's no longer in a position to blame Clinton for the economy, especially after the record boom while Clinton was in office. Like it or not, Clintonomics worked, Dubyanomics doesn't.
It's still the economy.
We're mired in an unpopular war.
Don't forget healthcare.