Why is it a bad assumption? He's at an all time low approval rating and the election was a 2% differential. That doesn't seem like a "Very bad assumption" to me. Don't forget this election drew the largest amount of voters in history.
I agree the economy is doing better even though there are some serious longterm problems but most people don't vote or form their opinions of the admin in office based on longterm problems which is why it should be very worrying for Republicans that with economic indicators up that Bush is at his lowest poll ratings ever.
I agree that this is a bad assumption because according to a lot of polling leading up to election day Kerry should've won. While Bush's ratings are down now its no like they plumetted since the election. I have to hand it to the Republicans because they won based on organization. They effectively countered the Democratic get out the vote effort with a superior one and effectively exploited a range of narrow hot button issues. At the same time the Dems couldn't motivate a greater number of younger voters to come out and vote which would've tipped the balance. As one Kerry strategist said on Nov. 1, 2004 "If you see a lot of young people waiting in line at polling stations then we are going to win." That didn't happen and the under % of the under 25 vote barely rose above previous elections.