Thomas Balcerski teaches history at Eastern Connecticut State University. He is the author of "Bosom Friends: The Intimate World of James Buchanan and William Rufus King" (Oxford University Press). He tweets @tbalcerski. The opinions expressed in this commentary are his own. View more opinion at CNN.
(CNN)With the 2020 presidential election less than a week away, the prospect once more looms of a challenger unseating an entrenched incumbent president. If Democratic candidate Joe Biden were to win when the results are finalized, he would make Republican President Donald Trump just the 11th incumbent in American history to try, but fail, to secure reelection.