Paul Michael, noting the troubles that President Bush is having in advancing his ambitious agenda, writes interestingly on the curse of second presidential terms, saying that it goes back to at least Richard Nixon.
But second term problems go way back before Nixon. It was during George Washington's second term that the terminally creepy Thomas Jefferson and his acolyte, James Madison, went full-tilt with an anonymous assault on the first president. While Washington continued to serve effectively, he was considerably hampered by Jefferson's cowardly attacks.
FDR, though elected for third and fourth terms, had a tough time during his second round. This was somewhat self-inflicted, owing in part to his Court-packing scheme. Nonetheless, his second term was problematic.
Two reasons that second terms have been bad: (1) The second-term administration grows weary; (2) The administration suffers from a sense of entitlement to power. Second terms are when indictments and scandals are likely either to happen or catch up to administrations.
A reason for second term troubles of more recent vintage is that under the Twenty-Second Amendment, Presidents are term-limited. This means that on election to second terms, they immediately attain lame duck status. This leads to a rather rapid erosion of presidential power. Yes, they still possess all their Constitutional powers. But they suffer from major "clout leak," losing their capacity to make those who cross them miserable. They're put on less sure footing with a Congress, a Court, and a bureaucracy which have no term limits.