Several emails, most of them nominating (or arguing against) 1980 and 1888.
Here’s one email: “The three incumbents to lose since 1936 – Ford, Carter and H.W. Bush – were seriously challenged from within their party in primaries, fighting to regain up to 33 points in late summer and behind through most of the fall (Carter the Gallup exception for the third circumstance). Each of them faced considerable problems with charisma or the appearance haplessness; for all his detractors, Bush is grudgingly accepted as irrepressible and earthy. Carter, Reagan and Clinton ‘connected’ with the American public. Kerry’s inability to do so has probably won more headlines than anything else.”
Another emailer points out that there are more examples of the reverse: “incumbent presidents everyone more or less thought would lose but didn’t,” like Harry Truman.