Conrad Black writes in his biography of FDR: "Most Americans disapproved in principle of the idea of three straight terms." But the perilous times facing the world, and the skill with which FDR had foreseen and managed the runup to war and the beginnings of US involvement made them more comfortable with setting aside principle.

And FDR never actually announced he was seeking a third term; instead he accepted a draft of the convention. In fact, he had engineered the draft behind the scenes, and frankly there was no one else qualified to be the Democratic nominee in 1940. And he knew better than anyone how much his political and tactical skills would be needed to bring the US into the war, at the right time, to defeat Hitler.

His acceptance speech to the 1940 Democratic convention uses the language of being called to serve:

Lying awake, as I have, on many nights, I have asked myself whether I have the right, as Commander-in-Chief of the Army and Navy, to call on men and women to serve their country or to train themselves to serve and, at the same time, decline to serve my country in my own personal capacity, if I am called upon to do so by the people of my country.

As Black writes, “It is a measure of Roosevelt’s great prestige and oratorical talent that he managed to put such a pious trumpery over without convulsing the country in mocking disbelief.”