And for those who like their stuff quantifiable, poli sci guy Ed Tufte (who has since become something of an "information design" guru) offered numbers supporting above: since WW2, when economy is "doing well," incumbent party does well. Tufte's _Political Control of the Economy_ - positing "economic- electoral cycle" in which approaching election, for various reasons, induces "artificial" economic growth obtained via government economic policy instruments - runs through 1976 but others have continued up-dates through 1996.
Morris Fiorina and some other poli sci guys cite above as most-often important factor in "retrospective voting" (Fiorina, _Retrospective Voting in American Natioonal Elections_). Michael Hoover