Incumbency is an advantage unless (1900 on, implying also chances of an incumbent running for a third term for which he is Constitutionally eligible )
1. one's party splinters (1912)
2. one's grand dream of foreign policy implodes (1920)
3. the economy collapses (1932)
4. a war goes badly (1952, 1968)
5. one is an incompetent campaigner for President (1976)
6. a gross embarrassment happens in foreign policy (1980)
7. one has no idea of what to do in a Second Term (1992)
I see multiple reasons for Donald Trump to lose a re-election bid: extremism, corruption, abuse of power, and perhaps a trade war that goes badly.
Ford ran probably the best losing campaign by any candidate since maybe Hughes in 1916.
