Similarly, even if Donald J. Trump’s and Ben Carson’s poll numbers begin to tank, why should they drop out soon? Do they have something better to be doing?

Money is also an important consideration; if no one’s funding you, it’s obviously hard to maintain a staff, travel to campaign appearances and the like. You can run a campaign on the cheap — Rick Santorum has become an expert at this — but it’s usually pretty hard to turn that into a successful nomination.

On the other hand, some recent changes to campaign finance laws are altering this calculation. As we saw in 2012 and this year, super PACs can spend huge amounts of money on ads, even if they can’t directly keep the campaign afloat. This creates what one observer has called “zombie candidates”: those who are still walking around long after their campaigns would normally be considered dead. That worked for Newt Gingrich in 2012, and we still might see this sort of activity this year as eccentric billionaires pick their favorites. This doesn’t necessarily change the final outcome of the nomination race, but it does prolong it.

Even those worried about temporary polling or fund-raising problems, like Jeb Bush, can look to the relatively recent example of John McCain’s 2008 run. The summer before the primaries, Mr. McCain’s campaign was flailing, running well short of its fund-raising goals, dropping in early primary polls and losing top staff members. Of course, he went on to win the nomination, although few would have blamed him if he’d dropped out the previous fall.

So when will the bulk of this year’s candidates drop out? Look back to the tumultuous 2012 cycle for the Republicans. Despite her campaign’s having seen its peak achievements in the summer of 2011, Michele Bachmann didn’t formally withdraw until early January 2012, after a poor showing in the Iowa caucuses. Herman Cain was the front-runner in the fall, but his campaign collapsed by late November, and he withdrew in early December.

The other temporary front-runners and top candidates from that year — Rick Perry, Rick Santorum, Newt Gingrich, Ron Paul — all stayed in until at least the New Hampshire primary, even if it had become clear for some of them that their campaigns were going nowhere. Of the major candidates that year, no one dropped out until December, and all but a handful were gone shortly after the New Hampshire primary in January. The winnowing didn’t start until late, but then it proceeded quickly.

We can expect a similar pattern this time around. Until the voting begins, most candidates will probably still maintain some belief that it will all come together for them. In addition, with polling in such a state of uncertainty, some candidates will stick around until Iowa and New Hampshire make it painfully clear that a bid is over. We can expect those with more to lose — current governors and senators — to be the first to drop out when it becomes clear that there is no path to the White House. Others may stick around longer, but not much.