Mitt Romney will not suffer Harold Stassen’s fate. But his decision not to seek the presidency in 2016 raises an interesting question: When did it become conventional wisdom that there is no second (or, at least, no third) act in presidential politics?

Henry Clay, whom Abraham Lincoln called his “beau ideal of a statesman,” ran for president four times. No one remembers him as a joke. William Jennings Bryan was a three-time Democratic presidential nominee. Also not a joke. Adlai Stevenson, twice nominated. Hubert Humphrey, Stassen’s fellow Minnesotan, ran three times. Ronald Reagan lost the GOP nomination in 1968 and 1976 before his victory in 1980. Definitely not a joke. Richard Nixon: lost in 1960, won in 1968. A joke, but for other reasons.

Today, it is nearly inconceivable that serious politicians can run multiple times for the presidency, especially after losing a general election. Every four years, the Mike Huckabees and Rick Santorums reemerge, but their campaigns are usually about something other than winning the presidency—building a personal brand, perhaps, or sending a message. The real contenders—those with a plausible path to the White House—don’t get a permanent free pass. This relatively new, unforgiving rule is partly a reflection of the presidency’s growing power since the 1930s, but it is also a product of how the nominating process has evolved. Until 50 years ago, a small number of big-state political bosses tightly controlled the selection of presidential nominees. In the late 1970s, all of that changed. The rising influence of television increasingly made politics resemble entertainment, while the fallout of the Vietnam War and civil rights movement shattered the authority of political bosses and elite political institutions. Out of this disruption came the system we know (and love, and loathe) today—the four-year presidential horse race, the campaign reality show, Iowa and New Hampshire, Super Tuesday, a nauseating array of debates and candidate forums.