Example: the linked graph shows the Dow Jones Average since 1900. Obviously, the DOW is not the only indicator of economic or Presidential success, but it's part of the picture. In 1980 Reagan inherited about 15 years of going nowhere. Then, 3/4's of the way through his first term, things started climbing rapidly and didn't stop until the tech stock bubble burst in the last year or so of the Clinton administration (followed shortly by 9/11). So, Reagan gets credit for incredible financial gain. Bush1 didn't last long in office, but nobody blames him for much economically because prosperity continued. Clinton is credited, too, because growth continued under his watch (again, until the last year). Then, starting in Clinton's last year it trended downward severely until 2002 when it headed up again.
So, the DOW turned around under Bush2's 1st term and he was re-elected. Things tanked the last year of his term (similar to Clinton's), and now it's turning around again.
The problem is that it takes awhile for prosperity to return even after the DOW recovers. President Reagan, Bush, and Clinton are all ok with everyone because they prospered (despite the downward trend starting under Clinton). Bush gets bashed because people didn't really get back to prosperity, and Obama will get bashed too if the DOW doesn't get back up to where it was when he took office and continue to sustain growth.