Wartime incumbents are incredibly weak, historically.
If you think Kerry is what made it close, you're clueless.
This is true, although only with unpopular wars like Iraq.
Pretty much any fullscale war.
War of 1812: Opposition party imploded
Civil War: Lincoln was expected to get slaughtered until a series of victories boosted his re-election prospects; even then, they didn't like Lincoln, and voted for him primarily because he represented Grant and Sherman
World War I: Wilson already re-elected, but his party lost control of the Congress in 1918, and the GOP landslided their way through the 1920s
World War II: FDR only got 54% against a young and untested opponent. Granted, I think this is more FDR fatigue than anything else.
Korean War: Truman declined to run for re-election. Maybe the, like, mid-30s approval rating got to him.
Vietnam War: Johnson ducks Nixon
