An incumbent winning re-election almost always happens. It is expected. It is like a tennis player holding serve.Bwa-ha-ha-ha-ha! Talk about living in a dream world -- Dwight Eisenhower was the first Republican to win reelection in the 20th century (TR was only elected POTUS once). In fact, Eisenhower was the first Republican to win reelection since GRANT. The next Republican to accomplish this and finish his second term was Reagan. And that's it in the 20th century. During that same period of time, Wilson (who won with less than 50% his first time up) and FDR both won reelection; Truman was only elected POTUS once; JFK was assassinated; LBJ was only elected POTUS once, and dropped out in 1968, when he might have (probably would have) won reelection and become the only President other than FDR to serve more than eight years; Carter was a one-term loser; Clinton won reelection.
I believe the author was writing of post WWII.