Since 1900, when an incumbent President ran (whether he had been elected in his own right four years before, or had ascended to the Presidency vie death or resignation), the incumbent won 12 times, and lost 4 times.
The four times the incumbent lost:
1912: Incumbent William Howard Taft finished third when former President Theodore Roosevelt (same guy who started this thread?) ran as a third-party candidate and split the GOP vote, allowing Woodrow Wilson to win easily.1932:FDR easily ousted incumbent Herbert Hoover, who was unjustly blamed for the Depression, and still is to this day by Hillary, et. al.
1976: Gerald Ford, who assumed ofice after Nixon resigned in disgrace, came from 20%+ down to make it close, but was defeated by Jimmy Carter.
1992:In the only one of the four incumbent defeats without an obvious reason, Bill Clinton ousted George Herbert Walker Bush.
So three of the four incumbent defeats took place under truly extraordinary circumstances. In the fourth, 1992, several conditions, none of them likely sufficient alone to defeat GHWB, unfortunately came together: an economic slump, Perot, and an odd detachment by the incumbent in the face of attacks. Based on history alone, I like Dubya's chances. If the Democrats further lengthen the odds against them by nominating, say, Howard Dean, so much the better.
Hey wait...what about Clinton? He was fond of LBJ's.