Joshua Spivak is a senior fellow at the Hugh L. Carey Institute for Government Reform at Wagner College in N.Y. He blogs about government at http://recallelections.blogspot.com/.

After the 2012 campaign, Mitt Romney insisted he wasn’t going to run for president again. Now, a 2016 run appears likely but probably won’t be successful. (Manuel Balce Ceneta/AP)

Taking heart to the saying “third time’s the charm,” Mitt Romney has shaken up the nascent 2016 presidential race with his announcement that he may not be done seeking the presidency. The electorate may be tired of Romney, but his persistence is somewhat presidential: A quarter of presidents ran unsuccessfully for the office at least once before winning. From Thomas Jefferson to George H.W. Bush, there is a long history of losers coming back to claim the White House.

Still, Romney’s supporters should have good reason to be worried that his third time could be another flop.

What separates Romney from other comeback presidents is that he’s already received his party’s nomination and lost once before. The recent comeback kids did not receive the nomination in their first runs for office. For example, both Ronald Reagan and George H.W. Bush came in second in their earlier attempts for the nomination. Reagan, probably the most noteworthy candidate who ran more than once, boosted his name-recognition and his credibility with the party’s conservative base in his first two runs, especially when he almost toppled sitting President Gerald R. Ford in 1976.

But once you look at the candidates who received the nomination, lost the general election and ran again, the road back to the White House appears much tougher. The last person to lose as a nominee and then go on to win the presidency — or even to get his party’s nomination more than once — was Richard Nixon, who lost the election on a razor-thin margin in 1960 and then won triumphantly in 1968. Before that, it was fairly common for a party to renominate a candidate. William Jennings Bryan was the Democratic nominee three times and never won; Adlai Stevenson got the Democratic nomination twice in the 1950s; Thomas Dewey was the unsuccessful Republican nominee in 1944 and 1948. But all of these candidates share something Romney lacks: Their campaigns occurred before the advent of the current primary and caucus system for choosing a nominee. These earlier nominees needed only to appeal to the narrow support of a political convention.

The modern primary and caucus system, which has been revised after each quadrennial election, gained ascendancy in the 1970s. Since then, not only has no politician come back after winning a nomination but losing the general election, none have really tried. Even when it is discussed, as it was with Ford, Al Gore and John Kerry, the candidates later thought better of the idea. What has happened instead is a weird split between whom the two parties select.

The Republicans have regularly nominated the guy who effectively came in second in the previous race — this happened with Reagan, Bush, Dole, McCain and Romney. (Romney technically came in third in 2008, but that was due to his early, post-Super Tuesday exit once he saw victory was out of reach.) This suggests that past candidates are helped by running a strong race, yet knowing enough to drop out and rally around the party’s standard-bearer once the nomination appears lost. These silver-medalists seem to gain instant credibility within the party for their strong, but losing runs.

The Democrats seem to have a very different standard. Since 1972, Al Gore is the only nominee to have previously run for president. (Gore gained a huge leg up by serving as vice president first.) While all signs point to Hillary Rodham Clinton changing this trend, recent history shows that Democrats appear to want to see a fresh slate of presidential contenders.

There’s no clear reason for this discrepancy, although the fact that Republicans have been very interested in nominating their second-place finishers their second time around may be enough to get first-time losers to run again.

This recent history, which rejects presidential nomination reboots, is probably based on the primary system itself. The old convention system rewarded nominees who were plugged in to the old system and who had the support of a core base of party leaders. The transactional nature of the convention itself meant that it was easier to deal with the known quantity, especially since the state party leaders may have already made deals with the candidate and his advisers in a previous race. It made sense for leaders to go back to the same well, even if it may have meant running a loser a second time.

But candidates now need to appeal to a very different universe of electors in the primary system. The past nominee has to overcome the stigma of losing — and, as polls show, the party faithful are quick to turn on a loser. Primary voters, always looking for a winner, are happy to ignore the last loser and seek a new man or woman for the job. This fact may be why few candidates attempt a second run these days. It’s not that the candidates have lost the desire or drive. It’s just that they can see that, in addition to all the other hurdles, they now have to overcome the “loser” label.

Mitt Romney and his supporters may be excited by the examples of comebacks among presidential candidates. But those happened in a much earlier, different time. Having to overcome the stigma of leading his party to a 2012 loss might be a game-ender.