If Michael Bloomberg is looking for evidence that a late entrant can win a presidential nomination, all he has to do is look back. About seven decades.

It happened in 1952, when Illinois Gov. Adlai Stevenson—after repeatedly refusing President Harry Truman's requests to jump into the race as Truman's Democratic successor—waited until the convention began to declare himself a candidate. Stevenson gave an acclaimed welcoming speech to delegates in Chicago, changed his mind, and won the nomination on the third ballot.

Bloomberg's surprise late entry into the 2020 race is a familiar story: The candidate who casts himself in the role of The Savior Who Is Waiting in the Wings is a quadrennial feature of presidential campaigns. For decades, Democrats and Republicans alike have persisted in looking beyond the field of those desperate few who have spent months, if not years, racking up frequent flier miles; eating indigestible food; begging for money; and crowding into coffee shops, union halls and living rooms trying to build a constituency. Right around this time of the cycle, these voters and pundits and party operatives reliably hit the panic button, certain that somewhere above the fray stands a candidate free of the now-obvious flaws that burden the rest of the field. It’s a yearning that invites comparison to the Groucho Marx quip about refusing to join any club that would have him as a member: “I refuse to support any candidate who overtly is seeking my vote.”

There’s one pesky fact about these late-entry candidacies: They never succeed. Only once have they even materially affected the outcome of a fight for the nomination.

Stevenson's success—and he didn't even win the presidency—came at a radically different time in American politics. In 1952, virtually no delegates were chosen in primaries; the party’s power brokers and bosses came to conventions with the ability to actually decide who the candidate would be—in a smoke-filled room, if necessary. We are in a different political universe today, and have been for the better part of 50 years.